Quote posts missing quote

This patch fixes a couple of problems with quote post type handling.

* Fetched quote posts were missing :source_url, :source_title and :text. I've added those.
* Posting a quote didn't allow setting of source_url and source_title. Tumblr accepts those, so I've added them
* Make the body of quote posts map to :source, rather than :quote. This change may be controversial, but I think this better because :source can contain markdown or html, and it can be longer than :quote. It's also consistently named -- quote text is called :quote in Tumblr's posting API and :text in retrieval.