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 remove them, we'd require users on pre-connected envs to invoke the gcloud module with no arguments.

So, this wouldn't work:

```js
var gcloud = require('gcloud');
var bucket = gcloud.storage.bucket({});
```

Instead, they'd have to:

```js
var gcloud = require('gcloud')();
var bucket = gcloud.storage.bucket({});
```