Add OkHttp driver to support modern HTTP verbs. DO NOT MERGE!

Add OkHttp driver to support modern HTTP verbs.

Using this driver, all mock and live tests for the OpenStack Marconi API pass, including the ones that use PATCH requests with a body. However, the integration tests for the driver itself fail randomly.

An example test output can be found here: https://gist.github.com/nacx/7953970

DO NOT MERGE THIS PULL REQUEST! It is just made to help debugging the failing tests. Once all pass, this will be closed and a proper one will be made.