Updates to README and storage

Author: auser

README.md

@@ -589,6 +589,47 @@ storageRef.downloadUrl()
 })
 ```
 
+#### download()
+
+It's possible to download remote files as well. The `download()` method will take a remote file and download and save it to the user's device. It is implemented on the `storageRef`:
+
+```javascript
+const storageRef = data.firestack.storage.ref('photos/photo.jpg');
+const localPath = `downloadedFile.jpg`;
+storageRef.download(localPath, (msg) => {
+  // downloading state callback
+})
+.then(res => {
+  // res contains details about the downloaded file
+})
+.catch(err => {
+  // error contains any errors in downloading
+});
+```
+
+The method accepts a callback that gets called with any download events:
+
+* download_progress ({eventName: 'download_progress', progress: float });
+* download_paused ({eventName: 'download_paused'})
+* download_resumed ({eventName: 'download_resumed'})
+
+As helpful constants, Firestack exports a few storage constants on the `firestack.constants` getter:
+
+* MAIN_BUNDLE_PATH
+* CACHES_DIRECTORY_PATH
+* DOCUMENT_DIRECTORY_PATH
+* EXTERNAL_DIRECTORY_PATH
+* EXTERNAL_STORAGE_DIRECTORY_PATH
+* TEMP_DIRECTORY_PATH
+* LIBRARY_DIRECTORY_PATH
+
+And we also export the filetype constants as well:
+
+* FILETYPE_REGULAR
+* FILETYPE_DIRECTORY
+
+> Note: this idea comes almost directory from [react-native-fs](https://github.com/johanneslumpe/react-native-fs), so we don't claim credit for coming up with this fantastic idea. 
+
 ### Realtime Database
 
 The native Firebase JavaScript library provides a featureful realtime database that works out of the box. Firestack provides an attribute to interact with the database without needing to configure the JS library.

android/src/main/java/io/fullstack/firestack/FirestackStorage.java

@@ -1,8 +1,11 @@
 package io.fullstack.firestack;
 
+import android.os.Environment;
+import android.os.StatFs;
 import android.content.Context;
 import android.util.Log;
 import java.util.Map;
+import java.util.HashMap;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.InputStream;
@@ -41,6 +44,17 @@
 class FirestackStorageModule extends ReactContextBaseJavaModule {
 
   private static final String TAG = "FirestackStorage";
+  private static final String DocumentDirectoryPath = "DOCUMENT_DIRECTORY_PATH";
+  private static final String ExternalDirectoryPath = "EXTERNAL_DIRECTORY_PATH";
+  private static final String ExternalStorageDirectoryPath = "EXTERNAL_STORAGE_DIRECTORY_PATH";
+  private static final String PicturesDirectoryPath = "PICTURES_DIRECTORY_PATH";
+  private static final String TemporaryDirectoryPath = "TEMPORARY_DIRECTORY_PATH";
+  private static final String CachesDirectoryPath = "CACHES_DIRECTORY_PATH";
+  private static final String DocumentDirectory = "DOCUMENT_DIRECTORY_PATH";
+
+  private static final String FileTypeRegular = "FILETYPE_REGULAR";
+  private static final String FileTypeDirectory = "FILETYPE_DIRECTORY";
+
 
   private Context context;
   private ReactContext mReactContext;
@@ -186,4 +200,34 @@ public void onPaused(UploadTask.TaskSnapshot taskSnapshot) {
       callback.invoke(err);
     }
   }
+
+  // Comes almost directory from react-native-fs
+  @Override
+  public Map<String, Object> getConstants() {
+    final Map<String, Object> constants = new HashMap<>();
+
+    constants.put(DocumentDirectory, 0);
+    constants.put(DocumentDirectoryPath, this.getReactApplicationContext().getFilesDir().getAbsolutePath());
+    constants.put(TemporaryDirectoryPath, null);
+    constants.put(PicturesDirectoryPath, Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES).getAbsolutePath());
+    constants.put(CachesDirectoryPath, this.getReactApplicationContext().getCacheDir().getAbsolutePath());
+    constants.put(FileTypeRegular, 0);
+    constants.put(FileTypeDirectory, 1);
+
+    File externalStorageDirectory = Environment.getExternalStorageDirectory();
+    if (externalStorageDirectory != null) {
+        constants.put(ExternalStorageDirectoryPath, externalStorageDirectory.getAbsolutePath());
+    } else {
+        constants.put(ExternalStorageDirectoryPath, null);
+    }
+
+    File externalDirectory = this.getReactApplicationContext().getExternalFilesDir(null);
+    if (externalDirectory != null) {
+      constants.put(ExternalDirectoryPath, externalDirectory.getAbsolutePath());
+    } else {
+      constants.put(ExternalDirectoryPath, null);
+    }
+
+    return constants;
+  }
 }