Force Android 4.0 as minimum for now

This way we get a nice ActionBar. Support for older Android versions might
be added back in the future.
Improve display of the header by moving the date to the right
Make the App no longer crash when rotating the device
Keep date across orientation changes
Simplify watching/showing by using largely similar renderings
Use the latest gradle plugin
Improve the layout of the chat entries

The try to mimic a card layout.
Unify implementations of the adapeters
Use the ViewHolder pattern to improve performance of the ListView