First pass at stubbing out gcloud.pubsub.

Here's an initial pass at the gcloud.pubsub classes and methods... Thoughts?

CC: @tseaver @dhermes @proppy @aliafshar 
I think at least docstrings would make the API clearer (the mapping from the spec is non-obvious to me, e.g. the 'message' and 'labels' arguments to 'publish_message').