The GUI no longer run from the charm directory.
Updated the GUI and (under pyJuju) the API agent
so that now the files are served from /var/lib/juju-gui.
That directory is also removed when the service is
destroyed. To do that, this branch introduces
the Backend.destroy method, called in the stop
hook.
QA: deploy the charm using this branch,
check everything is ok, check that the GUI files
are stored in /var/lib/juju-gui.
Then destroy the juju-gui service and ensure
that directory is cleaned up.
R=benjamin.saller
CC=
https://codereview.appspot.com/13309044