Pass selected listener dataset name.

This pull request adds the dataset name to the suggestion object and passes it to "typahead:selected" listeners. The rational behind this change is that might be useful to know which dataset a given datum came from if it was a remotely loaded. An example use case would be reloading the choice from the correct datasource on page reload within a stateful app. The dataset parameter is just an additional parameter sent to listeners and should not break anything. See #203.