proguard enabled, lint disabled, library gradle reps updated

* proguard enabled for both debug and release for now (it helps to identify proguard problems early)
* update library build.gradle to use the same android buildtool as the main project
* basic proguard.cfg that disables obfuscation (reported stacktraces should be unaffected) and has basic rules for ActionBarSherlock projects

Overall it cuts the APK size pretty significantly (3.4MB -> 2.6MB).