Related to #1102

I just tried to walk a group of people through gcloud.pubsub, and of the few who found the docs under "Pubsub usage", almost everyone was frustrated by the examples being there rather than next to the method they were interested in. They couldn't find the specific thing they wanted quickly, whereas finding the right methods was pretty straight forward -- but then example code was hidden in a big long (and separate) page.

Ideally we would:

1. Move all the usage examples to live next to the methods they describe
2. Remove the pubsub-usage.rst file
3. Have test coverage of those docstring examples

(ie, https://googlecloudplatform.github.io/gcloud-python/stable/pubsub-topic.html has no example code snippets)