Added basic channel subscription and feed pages (#620)

Added basic channel subscription and feed pages

- Room Persistence for sqlite support.
- RxJava2 for reactive async support.
- Stetho for database inspection support.
- Enabled Multidex for debug build.

app/src/main/java/org/schabi/newpipe/App.java

@@ -3,6 +3,7 @@ package org.schabi.newpipe;
 import android.app.Application;
 import android.content.Context;
 
+import com.facebook.stetho.Stetho;
 import com.nostra13.universalimageloader.core.ImageLoader;
 import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
 
@@ -64,6 +65,8 @@ public class App extends Application {
                             "Could not initialize ACRA crash report", R.string.app_ui_crash));
         }
 
+        NewPipeDatabase.getInstance( getApplicationContext() );
+
         //init NewPipe
         NewPipe.init(Downloader.getInstance());