Resolved Issue #22 - Added 'selection' callback and event

Added a selection callback:

```javascript
  $('.example-countries .typeahead').typeahead({
    name: 'countries',
    prefetch: '../data/countries.json',
    limit: 10,
    selectionCallback: function(query, selectedItem) {
      console.log(query, selectedItem);
    }
  });
```

Also, added a 'selection' event:

```javascript
  $('.example-twitter-oss .typeahead').typeahead({
    name: 'twitter-oss',
    prefetch: '../data/repos.json',
    template: [
      '<p class="repo-language">{{language}}</p>',
      '<p class="repo-name">{{name}}</p>',
      '<p class="repo-description">{{description}}</p>'
    ].join(''),
    engine: Hogan
  });
  
  $('.example-twitter-oss .typeahead').bind('selection', function(evt) {
    console.log(evt.query, evt.selectedItem);
  });
```