Use common base for UDisks wrapper classes

The base class has a cls.create classmethod that can be used as a
convenient constructor that will automatically take care of connecting
the dbus object.
Fallback on UDisks2 if UDisks1 can not be connected

In the case that no version is explicitly requested it seems appropriate
to choose whatever we find.
Merge pull request #11 from coldfix/fallback-to-udisks2

Fallback to udisks2