implement HttpResponseCodes for RFC 4918

RFC 4918 is a set of WebDAV response codes detailed in https://tools.ietf.org/html/rfc4918 . Specifically, there are five:

* 424, Failed Dependency
* 507, Insufficient Storage
* 423, Locked
* 207, Multi-Status (for compound requests and sub-requests)
* 422, Unprocessable Entity

I specifically need only 422, but it seemed to make more sense to implement the whole RFC.
Merge pull request #9 from stackmob/f/rfc4918

implement HttpResponseCodes for RFC 4918