Issue 42

This solves the infinite loop issue https://github.com/ACRA/acra/issues/42#issuecomment-12134144 but only for apps running on Android 4.0 or later.

It essentially listens for Activity lifecycle events and keeps track of the last Activity that was created. And on crash explicitly finishes that Activity before killing the Process (which will attempt to recreate the entire Task stack).

To be available on versions < 4.0 we need some way of working out which Activity was last created.