Add a method to unwrap the target API from the view

See the comments in the parent issue.

Add an unwrapApi() method to get at the underlying Api from a View. For example:

ComputeServiceContext context = ContextBuilder.newBuilder("rackspace-cloudservers-us")
.credentials(username, apiKey)
.buildView(ComputeServiceContext.class);
ComputeService compute = context.getComputeService();
NovaApi novaApi = context.unwrapApi(NovaApi.class);