Add support for jsonp

[codingluke]

Example

$('#searchfield').autocomplete({
    url: 'http://www.someexternalurl.com/autocomplete',
    remoteDataType: 'jsonp',
    jsonpCallback: 'customCallbackName'
});

Response must be encapsulated in a function defined with jsonpCallback

customCallbackName({"results": ["suggestion1", "suggestion2", ...]})