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.
Datastore is the exported value for this module. That's how we can create an encapsulated instance from lib/index.js. The gcloud module works the same way, so it can be dual-purpose: exposing the static methods + create isolated instances.