Add smack-android and redesign SASL authentication

This commit marks an important milestone with the addition of the
smack-android subproject. Smack is now able to run native on Android
without requiring any modifications, which makes the aSmack build
environment obsolete.

It was necessary to redesign the code for SASL authentication to achieve
this. Smack now comes with smack-sasl-provided for SASL implementations
that do not rely on additional APIs like javax for platforms where those
APIs are not available like Android.