datastore: scope Key creation to include NS. fixes #116.

RE: #116

I actually have 2 different implementations for this, but this was the one I thought was the most logical.

When a user creates a dataset, they can provide a namespace. Then, anytime they need a key, instead of `datastore.key()`, they call `datasetInstance.key()` - the key will be created with the namespace already bound to the Key.

The other implementation removed any association of a namespace from a Key object, which [felt wrong](https://developers.google.com/datastore/docs/apis/v1beta2/key).

PTAL & LMKWYT.
The convention that I see is only: if it's meant to be of type object, but it doesn't exist (wasn't defined, wasn't provided, anything), it's `null`. If it's any other type that that variable could hold, it's `undefined`.

This is common:

```js
function Class() {
  this.something = null;
}

Class.prototype.getSomething = function () {
  this.something = 'Here you go';
};

// But this makes no difference:
Class.prototype.isThereSomething = function () {
  return !!this.something;
};
```

But outside of classes, this is not a common sight:

```js
var something = null;

function getSomething() {
  something = 'Here you go';
}

function isThereSomething() {
  return !!something;
}
```

Both `isThereSomething` functions work the same way, whether `something` began as null, or wasn't assigned a value (resulting in type `undefined`).

The only point in using `null` is when an object is expected is to check "was a value of any type assigned to this".

```js
var user = null;

retrieveUser(function (res) {
  // res = {}
  user = res.user;  
});

function wasUserRetrieved() {
  return user !== null;
}

wasUserRetrieved();
// true
```

It's not truly important in JavaScript. I just follow conventions. You really don't see null a lot out there, and I doubt many even know to trace the null vs undefined trends they see back to Zakas's definition (or maybe even something else I don't know!).

Since `null` is common in classes, I'm fine to go forward with that.