unity: add service file for unity panel

Unity in saucy switched to using upstart instead of dbus for launching unity-panel-service, which is responsible for handling indicators like datetime, logout, etc. Since we don't have upstart, this broke the panel in Unity under crouton.

This patch adds a service file so that unity-panel-service can be launched by dbus again.

Fixes #446.

Relevant commit from unity: https://code.launchpad.net/~ted/unity/upstart-panel-service/+merge/165739