Add test for udiskie.common: DBusProperty, DBusProxy

These tests are somewhat ill-defined and could fail on other platforms
if the requested services are not available. In this case they should be
improved to use only defined services.
Add some more documentation in udiskie.udisks
Export only entry-points from udiskie.cli by default

The other methods can still be explicitly exported, but they are not
part of the public API.