Added check of ROVI_API_KEY and ROVI_SHARED_SECRET.

 - Only print track-image ( and surrounding markup ) when it's not None.
Return None in 'get_track_image' when r.status_code not ok.
Merge pull request #73 from petteraas/optional_rovi_track_image

Added check of ROVI_API_KEY and ROVI_SHARED_SECRET.