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/NewPipeDatabase.java

@@ -0,0 +1,34 @@
+package org.schabi.newpipe;
+
+import android.arch.persistence.room.Room;
+import android.content.Context;
+import android.support.annotation.NonNull;
+
+import org.schabi.newpipe.database.AppDatabase;
+
+import static org.schabi.newpipe.database.AppDatabase.DATABASE_NAME;
+
+public class NewPipeDatabase {
+
+    private static AppDatabase sInstance;
+
+    // For Singleton instantiation
+    private static final Object LOCK = new Object();
+
+    @NonNull
+    public synchronized static AppDatabase getInstance(Context context) {
+        if (sInstance == null) {
+            synchronized (LOCK) {
+                if (sInstance == null) {
+
+                    sInstance = Room.databaseBuilder(
+                            context.getApplicationContext(),
+                            AppDatabase.class,
+                            DATABASE_NAME
+                    ).build();
+                }
+            }
+        }
+        return sInstance;
+    }
+}