Add support for jsonp

Intelligent adding a jsonpCallback function. The function name can be customized with the option jsonpCallback

Example
```javascript
$('#searchfield').autocomplete({
    url: 'http://www.someexternalurl.com/autocomplete',
    remoteDataType: 'jsonp',
    jsonpCallback: 'customCallbackName'
});
```
Response must be encapsulated in a function defined with jsonpCallback
```javascript
customCallbackName({"results": ["suggestion1", "suggestion2", ...]})
```