Added check of ROVI_API_KEY and ROVI_SHARED_SECRET.

Added check of ROVI_API_KEY and ROVI_SHARED_SECRET.
 - Only print track-image ( and surrounding markup ) when it's not None.


From reading the README.md under examples/webapp I got the feeling that the album-art is meant to be optional,  but I got a error when trying to run the webapp without the required settings.

As I haven't signed up for Rovi I hope somebody who has done that can verify that it still works as before when the config settings are properly in place.


Error in 'master' is : KeyError: 'ROVI_API_KEY' on line 27 in index.py ( + a long stack )