Start notifying of new mail at session startup: Bug #714644

Geary can now be configured to notify of new mail at startup.  When
the user logs in, Geary will autostart with a hidden window and
notify of new mail as usual.  When Geary is formally executed by
the user the Geary window simply appears.

In this mode, if the user closes the window Geary will return to its
hidden state.  Quit must be used to close the process.