Add Signature to messages

This adds a custom signature option to Mms app,
which appends a custom string set in settings
to all messages send.

PS2 : Fix Test Derp

Credits: DroidJunk

Change-Id: I2b3572e47a183c18c70bf8ef4960d60695ac3c4b

Conflicts:
	src/com/android/mms/ui/MessagingPreferenceActivity.java