New project structure.

Introduce Gradle, as it is the prefered build system for new android projects and Android Studio. Also replace ActionBarSherlock by AppCompat which was introduced in Support Library 18.
Add settings fragment for pre honeycomb.

AppCompat misses a backward compatible PreferenceFragment implementation. By using a PreferenceActivity the action bar is not available on older OS versions. To always enable acton bar, a PreferenceFragment is used on newer versions and a static settings fragment (not generated from xml) on older version.
Remove ActionBarSherlock from AboutActivity.
Merge branch 'appcompat' into develop