jclouds OkHttp driver

A driver to use the OkHttp (http://square.github.io/okhttp/) client as an HTTP library in jclouds.

This driver adds support for use of modern HTTP verbs such as PATCH in providers and APIs, and also supports SPDY.

To use the driver, you just need to include the OkHttpCommandExecutorServiceModule when creating the context:

ContextBuilder.newBuilder("provider")
    .endpoint("endpoint")
    .credentials("identity", "credential")
    .modules(ImmutableSet.of(new OkHttpCommandExecutorServiceModule()))
    .build();