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 
This import is the only difference between this and the equivalent [functions][1] in storage and datastore land.

We'd be better off with a helper function which accepts the connection class as an argument and then wrapping calls to that function here (and elsewhere).

Also, see https://github.com/GoogleCloudPlatform/gcloud-python/pull/235

[1]: https://github.com/GoogleCloudPlatform/gcloud-python/blob/55adeb3cf83e9714b3783d7ab5e56ae01ab69e25/gcloud/storage/__init__.py#L39