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 thought this, too. Overriding support comes from https://github.com/GoogleCloudPlatform/gcloud-node/issues/191#issuecomment-54849829

I think it's confusing for different connections, but possibly useful for projectIds. Though, I think if I was using gcloud to interact with multiple projects in one file, I would want to isolate them to unique gcloud instances, instead of piggybacking off one another.