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.
Datastore here refers to the module being exported from this module. dataset is on the exported object so that this works:

```js
var ds = require('gcloud').datastore.dataset({});
```