Use oEmbed #16

We're adding an oEmbed service to DocumentCloud, so I'll be adapting this plugin to take advantage of it. The syntax of the shortcodes won't change, so it'll be backwards-compatible.

Goals:

1. Fetch the embed code from our oEmbed service, natch
2. Support existing [documentcloud] shortcodes for documents, but prefer [dc-document] going forward to distinguish among target resources; option syntax remains the same
3. Add new shortcodes support for other resources: notes (Support embedding a single note #4), collections/searches (Support embedding collections/searches #3), and (eventually) pages
4. Add support for recognizing resource URLs on their own line, as described here; if Try to get on the official WordPress oEmbed whitelist #15 were accomplished, then this would only be enabled for versions of WP prior to that
5. Use this implementation as a meta guide for future plugin authors

Edited: Abandoned design of separate shortcodes for different resources. Going to instead parse URL and determine resource type from there.