Automate cleanup of global resources opened by Palaso assembly

At the moment, this applies only to Linux/Mono for one particular
resource, but could be extended as needed.
Fix a typo: "Palaso.UseDrive" should read "Palaso.UsbDrive"
Merge pull request #16 from sillsdev/AutomaticPalasoShutdown

Automate cleanup of global resources opened by Palaso assembly