Native libs missing in apk

Proposed fix for issue 372: https://code.google.com/p/maven-android-plugin/issues/detail?id=372

The issue was introduced by the following change: https://github.com/jayway/maven-android-plugin/pull/187

The above change required that any native library dependency use the artifact classifier to define the hardware architecture. Unfortunately it seems that the use of the classifier for this purpose is not well known and thus builds failed to include the corresponding native libraries.

My proposed fix maintains the above change but uses a default hardware architecture for artifacts that don't have a classifier. The fix also restores a configuration option to set the default that was removed by the above change.