Temporarly remove .travis.yml to avoid builds
Add Folder class to the Plug
Add Plug.folders
Add folder to Metadata and remove owners

The fid is now calculated from the current folder and the
filename.
Update Plug.get_metadata to handle folders
Metadata.path return the full path of the file
Fix folders initialization in the Refree
Referee: Fail gracefully if there is an error
Referee: Adapt handle_update to the folders