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.
To allow this:

```js
var gcloud = require('gcloud');
var ds = gcloud.datastore.dataset({}); // static method on datastore module.
```

We are still allowing configuration of a dataset at the time of creation, so not all uses will be pre-configured, like:

```js
var gcloud = require('gcloud')({ credentials: {} });
var ds = gcloud.datastore.dataset({}); // instance method on datastore module.
```

The static method looks simply like:
```js
Datastore.dataset = Dataset;
```

Would you rather it be written out:
```js
Datastore.dataset = function(config) {
  return new Dataset(config);
};
```
?