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.
The instantiation of a dataset can have other things than what is provided for global config, right?

https://github.com/GoogleCloudPlatform/gcloud-node/issues/191#issuecomment-54814383

Dataset and our other modules could grow to allow other options. We probably shouldn't rely on a user providing all the details in the global config that only make sense for a dataset.