Upgrade to OkHttp 2.1.0

This upgrades OkHttp to 2.1.0 and refactors the OkHttp driver to use its native methods. This PR could supersede https://github.com/jclouds/jclouds/pull/544 and fix [JCLOUDS-744](https://issues.apache.org/jira/browse/JCLOUDS-744).

Note that the integration test suite in the OkHttp driver is the same than the one for the default HTTP driver, so the coverage is pretty good. I've also executed the OpenStack Marconi live tests (Marconi uses the OkHttp driver by default) and only one failed but it is unrelated to the change.

This is just an initial implementation of the driver using the 2.1.0 version and the OkHttp native API, that should put us in a better place to address the commented improvements to the SSL connections. The current implementation is based on the default HTTP driver, and there will be room for improvement. In any case, it's a nice starting point :)

/cc @adriancole 
Sure! My plan is to have the driver working without changing the basic behaviour, and then address the SSL improvements in a separate PR. Sound good?