Video filtering

I had to make a couple additions for an app I'm working on, so thought I'd throw these back upstream. There's three changes:

1. Add the video_type field to the Video resource.
2. Add a custom 'filter' parameter, which is, uh, 'lightly' documented on the GB side but supports all sorts of cool stuff. There was already a `filter` method on the search class, so I did some hash munging on that method's input to keep backwards compatibility.
3. Add a `sort` parameter.

I can split these up into separate PRs if there's anything you don't like. Cheers!