feat(datastore): support setting a property indexed value

By default, property index values are set to `true`, without allowing
the user to specify an override. Now, when a user passes in an array
to `dataset.save`, they will have the option of setting `true` or
`false`.

Example:
```js
dataset.save({
  key: dataset.key('Company'),
  data: [
    name: 'propertyName',
    value: 'any value type',
    excludeFromIndexes: true
  ]
}, function(err, keys) {}).
```
Resolves: #208
Related: http://goo.gl/tKVvhP
Oh okay, if it's not on the proto object, does it return a value of null? Usually not on the object means undefined. Not really a big deal either way, just curious why it's that way.

``` js
{}.hello === undefined // ==> true
{}.hello === null // ==> false
```