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:

dataset.save({
  key: dataset.key('Company'),
  data: [
    {
      name: 'propertyName',
      value: 'any value type',
      excludeFromIndexes: false
    }
  ]
}, function(err, keys) {}).

Resolves: #208
Related: http://goo.gl/tKVvhP