jclouds OkHttp driver

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

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();