Oembed methods can take a url or id.
Add User#description_urls

Instead of providing direct access to entities, expose a method that
returns Twitter::Entity::Url objects, similar to how entities are
accessed on Status objects.

Closes #350.
Merge pull request #349 from bshelton229/oembed-url

Allow the oembed endpoints to take a url as well as a status id
Clean up after #349