New rackspace auth identity filter

For grins, I reimplemented the poorly named content-identity filters for Rackspace Identity.

I called it the "Rackspace Auth Identity Filter" so it would fit along the lines of the "IP Identity" filter, which uses your IP address as your user. 

This uses Rackspace Authentication parsing logic to get the username out of a myriad of requests.

It uses the builtin scala XML parsing library to read XML responses, and it uses play-json to parse the JSON responses in a similar manner.

Open to comments, but I think it's substantially less confusing than the existing filters, and it's got way less dependencies. The code is cleaner, easier to understand (even though it's in scala).

I think we can probably ship this as part of any release of Repose, not just the latest one.