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.
If we don't invoke with new then gcloud.datastore is a constructor `gcloud.datastore instanceof Function === true` when no config value is given by default. Or is this by design? Do we want them to invoke it with config then? I guess this makes sense... Just would like clarification here. :)