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/res/layout/fragment_blank.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    xmlns:tools="http://schemas.android.com/tools">
+
+    <include layout="@layout/main_bg" />
+
+    <include
+        android:id="@+id/error_panel"
+        layout="@layout/error_retry"
+        tools:visibility="visible"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_centerHorizontal="true"
+        android:layout_marginTop="50dp"
+        android:visibility="gone" />
+
+</RelativeLayout>