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
"Datastore method" >> "Datastore entity mutation" might better explain it but I also don't want to overload the end-developer which a bunch of non-sensical lingo. I think this is good. :+1: