Stringify item to JSON before assigning to data-value

I was attempting to implement autocomplete / typeahead functionality with a search web service that returns a JSON array of  hashes as the result and found that data-value attrib only holds a string. 
This change serializes the item using JSON.stringify and parses the selected item.

i,e. the following options
```
source: [{id:1,label:'One'},{id:2,label:'Two'},{id:3,label:'Three'}],
matcher: function(item) {return (item.label.toString().toLowerCase().indexOf(this.query.toLowerCase()))+1;},
highlighter:function(item){return item.label;},
updater:function(item){alert("selected item id " + item.id);return item.label;},
```