Adopting alternate headset detection method if available

Now using AudioManager's methods to detect presence of headset if API
level sufficient -
isBluetoothA2dpOn() and isWiredHeadsetOn(). These are marked as
deprecated in the Android API documents, but is suggested for use by
Google's own docs

(http://developer.android.com/training/managing-audio/audio-output.html).

Manifest modified to add the permission MODIFY_AUDIO_SETTINGS which is
required for isWiredHeadsetOn() to work properly.
Merge pull request #21 from pugwonk/master

Adopting alternate headset detection method if available