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, right. What I'm checking for null here is something that's been proto-buf'd. This is `proerty.value`:

```js
{ boolean_value: null,
  integer_value: null,
  double_value: null,
  timestamp_microseconds_value: null,
  key_value: null,
  blob_key_value: null,
  string_value: 'value',
  blob_value: null,
  entity_value: null,
  list_value: [],
  meaning: null,
  indexed: null }
```