Generate .aar file instead of .apklib.

Switched to the native .AAR format, as outlined in:
https://code.google.com/p/maven-android-plugin/wiki/ApkLib
-> android-maven-plugin can also consume .aar properly now, so  it's not a breaking change.

Further upped the compatibility-v13 library version to 21.0.1. 
It requires the user to compile the compatibility library using https://github.com/mosabua/maven-android-sdk-deployer first.

Note: Once google increases the compatibility-v13 version, new developers of this library most likely are required to update this projects reference to that version.