Added a __contains__ method.

Objects that act like dictionaries need __contains__ to be able to handle the "if <key> in <object>" syntax.
BackupMonkey now uses the new prefs system

Did some fairly majory surgery to get BackupMonkey to use the new Preferences system for most of its settings. Conspicuously did NOT migrate anything having to do with email; that'll be done differently later. Along the way figured out how to use distutils.spawn.find_executable to call tar in a platform-independent way.