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 
Shouldn't this fail? AFAIK `filter` only takes 2 arguments. I can't make this work on 2.7.

Also, use of `filter` is discouraged and list comprehensions are preferred?