support global configuration.

#191
#172 

Here we go!

http://stephenplusplus.github.io/gcloud-node/#/docs explains a lot of this for me :)

But, key points:

1. Support global configuration
2. Remove `new` use from dataset & storage

These changes went hand in hand.

Smaller changes:

1. Using a community extend module
2. Added a "Getting started with gcloud" to the top of the doc's module pages.
I left some "dev-only" jsdoc comments in the code, let me know if this is helpful in explaining: https://github.com/stephenplusplus/gcloud-node/blob/global-config/lib/storage/index.js#L60

Basically, the `datastore` and `storage` modules are similar to how they were previously, in that `require('lib/__module__')` will return the same methods, but the actual value returned from `require('lib/__module__')` is a function. The function has static methods on them, as well as prototype methods to support the one-time configuration use case.