Move to oEmbed

So we were using the `embedly` plugin, just to see how their setup worked. Meh.

Now, we have our own oEmbed plugin, and I think it will be sweet.

This PR updates the `video_embed` plugin for our use, and updates all posts in the doc with video to call the new plugin.

any thoughts @bschwartz @jbancroft @freerobby ??