Port to PyGObject from PyGTK

Remove the dependency on PyGTK.

On archlinux the package python-gobject/python2-gobject is now
required.
Force menu image
Port notifications to gi.repository.Notify
Add PyGObject based password prompt named 'builtin'

to eliminate the dependency on zenity.
Remove --password-prompt parameter

Now only use a built-in heuristic.

NOTE: there is at least one edge-case, that is not handled nicely:
when running udiskie in the background, there may be an asociated tty
but no nice possibility to retrieve user input.
Replace python-dbus with PyGObject

There is also a direct binding for UDisks in gi.repository.UDisks. But
this supports only UDisks2 and would have been more work to port to.
Remove dbus test

The test is using dbus-python which is not a dependency anymore, and not
easy to port. Furthermore, it tests rather trivial properties that
should already be ensured by the GObject library.
Remove obsolete sections from .travis.yml
Merge branch 'master' into pygobject

Conflicts:
	doc/udiskie.8.txt
Update CHANGES
Merge pull request #33 from coldfix/pygobject

port from pygtk to pygobject