Add support for a custom remotePreprocessor function & autodetect JSONP

Solves #8 

Adds remotePreprocessor option to the Dataset specifications, which can process the response of a remote server call, in case the response is not in the correct format.

The function takes the response as a parameter and returns an array of
strings or canonical datums. See [Datums](https://github.com/twitter/typeahead.js#datums) for more information

Also compares window.location with the remote url to determine whether JSONP is required for a successful request and sets the appropriate dataType for the $.ajax

Updated the test suite & Readme.md