Merge pull request #2 from danhantao/master

Undo/Redo UI
Added Undo/Redo feature.
Update pom.xml
Merge branch 'master' of
https://github.com/danhantao/realtime-android-playground into
danhantao-master

Conflicts:
	src/main/java/com/goodow/realtime/android/playground/MainActivity.java