Initial RTL layout flipping support

* update build target to 4.2 (level 17)
* adds android:supportsRtl attribute on application
* flipped version of 'send' icon
* use start/end properties to fix layout of progress indicator on category search

This flips most of the layouts, but doesn't appear to properly flip the ViewPager on the detail view. Unsure why.

Tested with Hebrew on Android 4.2 Nexus 4, and doesn't appear to explode English on 2.3. Yay!