Adding video player

I'll probably add some more testing before I push this, but since that will take a while, I'm going to push this part up for review now.  The main files to look at for this feature are `videoplayer.dre` and `playlist.dre`, and there are a lot of working examples in the `video.html` smoke test.

Note I'm not sure if the right/left/top/bottom/center helpers I added to `dr.view` should stay in the main class, so I've hidden them from documentation for now and I'm going to extract them out into a mixin (once we have mixins).  I found them incredibly helpful in connecting views together quickly and simply, so I want to keep them around.