Bug 851905 - [email] Back-end testing: switch to real content window from entirely shimmed, to xpcwindow driver from xpcshell

@lightsofapollo Here's the unit test stuff that uses a real window IMAP works, although you need to update your b2g-desktop to a build made today for the test_compose's devicestorage stuff to work.

It has not been merged to include James' deferred loading stuff or your stuff yet.  This is mainly to give you a chance to do some review early, etc.

Although actually I did just see some type of new quasi-intermittent issue in test_compose which might be timing issues; unsure.