TimeVault should be external hard drive conscious

When backing up on an external hard drive, TimeVault should be conscious that an external HD could be plugged out at any time. In this case TimeVault should suspend until the drive is plugged in again.

In version 0.7.5-1 actually this makes TimeVault to have a strange behaviour: in my case while when started the status icon says it's watching about 19.000 directories, if the drive is unplugged and plugged again, the status icon will say only about 200 directories are watched, and no actions are scheduled (even some file has changed, or the total snapshot is not complete yet).