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.
Because we're offering a suite of tools that should be capable of understanding we're connecting in the same way each time. This whole thing started with just providing credentials for global re-use, then morphed into other config options. I prefer just connection configuration, personally, but it's just as flexible to support other commonly re-used properties.

Just let me know what you want to support and I'll make it happen.