Code review comment for lp://staging/~kevinread/timevault/external

This branch is based of astrommes "timevault-external" branch Rev 44.
I added a HAL listener to the server. It will wait for the DBUS signal "DeviceAdded" and then re-check if our backup drive is present (if it is not present yet). As I do not plan on working on this branch a lot I propose merging it into astrommes excellent work. I hope to do more work on disconnecting from the SQLite DB so we can unmount the database media if it is not in use.