Add Xsession script to launch Unity indicators

Unity in trusty has been broken (see #679); this fixes it.

* Adds the new `ubuntu-session` package to the `unity` target for `trusty` and later
* Adds a new entry for Xsession.d that will launch `unity-panel-service` (`saucy` and later) and any indicators that launch via upstart (`trusty` and later).

This also eliminates the need for the dbus service file workaround that was implemented for `saucy`.