UIModel now unlocks all workunits when exiting.  Unlock method added
to WorkUnit.
Added directory property to WorkUnitProvider which UIModel can use to
get the current working directory.
Removed progress manager from UIModel constructor.
Added shutdown method to WorkUnitProviders which causes them to 
unlock any workunits they have prefetched.
Merge pull request #149 from drusk/unlock_prefetched_on_exit

Closes #143.