Thumbnail and duration

[dpizzuto]

Referred to issue 109 The original implementation was did with API v2 which are deprecated right now. I replaced the old implementation with the new that requires an API key generated from Google Api Console, this key must be specified at row 6 of MHGallery.m through the const string MHYoutubeApiKey. Really, only the duration need to be called via v3 API because the thumbnails are available without API call (a little trick). I know that’s a quick and dirty solution in fact, the rest part of library will have to be modified.