Don’t test YouTube Analytics too soon

For some reason, impressions from 5 days ago are not available.
Will investigate the issue, but the test is still valid with a different date.
Merge branch 'fix-failing-spec'
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