Make channel.videos access more than 500 videos

According to http://stackoverflow.com/a/23256768, YouTube API
won't provide more than ~500 search results for any arbitrary query.

This commit allows to overcome this limitation by splitting channel.videos
into multiple paginated queries that ask for videos in chronological order
and use the `publishedBefore` parameter to collect all the videos.

Closes #23
Merge pull request #24 from Fullscreen/channels-with-more-than-500-videos

Make channel.videos access more than 500 videos