Add support for HTML5 data attributes

Typeahead is unable to read HTML5 data attributes because of [this](http://www.w3.org/TR/html5/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes). The browser will automatically lowercase the attribute and therefore jQuery won't be able to find the following keys:

 - ttValue
 - ttDatum
 - ttDataset