Make it possible to revoke / delete a token from the client

The token auth login servlet should also provide the ability to remove tokens from the client. This is required for too reasons:

• make it possible for Nuxeo Drive integration tests run from the python client to clean up the tokens created as part of the tests executions

• make it possible to automatically cleanup Nuxeo Drive tokens when the user decides to unbind-server from the commandline or the system tray icon menu.