AssignmentGenericFiles controller tests
Added support for deleting files from Dropbox
Removed spec for deleting physical file
When the AssignmentGenericFile is deleted, the controller will have the responsability to delete the physical file and rollback the DB transaction if it fails
Updated DropboxGateway errors
Tests for AssignmentGenericFile controller's destroy action
Renamed stylesheet file to alfred.css
Fixed assignment file delete
Added dm-transactions gem