docs.

RE: #19

Did a sweep of the Datastore doc blocks, as well as laid the groundwork for docs to come. Thanks to [jsdoc2md](https://github.com/75lb/jsdoc-to-markdown), 5 files are generated in a new `docs` directory, one for each service/module: "Common", "Datastore", "PubSub", "Storage", and one for `lib/index`, which is generated as the default "readme.md" for the directory.

For a look at the rendered results, see https://github.com/stephenplusplus/gcloud-node/blob/docs/docs/datastore.md