Use requests.structures.CaseInsensitiveDict instead of dict for self.headers.

Python 2.x delivers a mimetools.Message with lowercased keys, Python 3.x
delivers a http.client.HTTPMessage with uppercased keys.
Merge pull request #129 from petteraas/event_headers_uppercase

OK - all done thanks
Setup make lint, added make lint to travis script target.
Added flake8 and pylint as dev requirements.
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.
fix pylint complaints