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.
This is exactly why there is no global configuration.

Global conf should be a plain object of credentials to be reused while constructing services. I don't know why passing a plain object a few times is painful.

``` js
var conf = {
  projectId: 'my-other-project',
  keyFile: '/path/to/other/keyfile.json' 
};

var ds = gcloud.datastore.dataset(conf);
var st = gcloud.datastore.storage(conf);
// etc
```