Bug 1020644: WebAPI Tests for MMS (Part One: Change existing common code...

... to support MMS)

First step in adding tests for MMS. Both SMS and MMS tests use MozMobileMessage; so change the folder name to "mobile_message"; change the common code module name to "mobile_message_test"; and update the existing common code so it can be used for both SMS and MMS tests. A couple of other minor improvements along the way.