Add video.advertising_options_set and .ad_formats

This commit allows content owners to retrieve the advertising options
of a video, and in particular to list the ad formats.

Updating the advertising options was already possible, but retrieving
the data is required in order to be able to incrementally add ad formats.
Merge pull request #96 from claudiofullscreen/ad-formats

Add video.advertising_options_set and .ad_formats