BackButtons: Updated AlbumViewModel
Includes two new properties to keep track of FolderID and InFolderID, for the back button to work.
BackButtons: Added back buttons
One on DisplayImage view, takes user back to Album view in the folder that the picture comes from.

The other on Album view when not in root folder, takes the user up a folder level.
Merge pull request #24 from jarradm/BackButtons

Added Back buttons