Fix the blocking issue #20 that prevents app to start on Android 4.2
Scenario's default AI difficulty can now be changed via configuration. Default AI difficulty is now easy by default (was normal for scenarios and novice for random games). These changes affect new games only.
Library upgrade: Android SDK and SDL.
Prevent installation on incompatible devices (Android version < 4.2).
Compile code for ARMv7 (it may improve CPU performances).