Swap out HTTParty with HttpClient

Why? This [link](http://bibwild.wordpress.com/2012/04/30/ruby-http-performance-shootout-redux/) and this [one](http://bibwild.wordpress.com/2012/04/30/httpclient-is-a-nice-http-client-forin-ruby/) point out good reasons. Performance, HTTP 1.1 persistent connections across threads, clean API, etc.

I've also fixed #20. I've swapped out checks for an `errors` hash key on the response and instead I check for a `200` status code. Awe.sm doesn't seem to use `201` for created, or `202`, `204` for updated/deleted, it seems to only use `200`'s. This gem should figure out a way to return errors though. Perhaps raise custom exceptions and set the error messages to the value coming back in the errors hash.