Fixes for CustomMessageBox.

Including:

Navigation from CustomMessageBox Dismissed event causes
NullReferenceException
http://phone.codeplex.com/workitem/10575

CustomMessageBox not working with SaveAppointmentTask
http://phone.codeplex.com/workitem/10712

Unanswered NullReferenceException with CustomMessageBox when returning
from Task
http://phone.codeplex.com/workitem/10760

CustomMessageBox: IsLeftButtonEnabled and IsRightButtonEnabled
properties don't affect the state of the buttons
http://phone.codeplex.com/workitem/10932

And other minor fixes.