Restored Android 2.x compatibility

Launcher PNG loading issue because of flag not automatically set by new build tool version
Improved networking code

- Error handling: Every generic exception like NullPointerException will indicate an error, so that the reason for a failed login can be determined (either wrong password/username combination or any networking exception)
- Code consistency: CommunicationInterface can only be instantiated via .getInstance(); a successful login will trigger a logout
- Android 2.2 compatibility: Patched jsoup because of missing String.isEmpty() in Java std library
Push version