Xml serialization for API

Mostly this removes a hardcoding of `application/json` that was added to make it easier to work with the API in a browser. The problem is that a browser does _not_ use a "default" encoding - it specifically requests `text/html,application/xhtml+xml,application/xml` so you always get crappy in-browser output. Added to this, Tastypie does not really support HTML but instead of raising the unsupported exception that it does for other formats, it returns a silly string to the browser. This patch works around it in a way that is better than what we did before, though not perfect.