Adds support for a custom remotePreprocessor function

Adds remotesPreprocessor 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 https://github.com/twitter/typeahead.js#datums for more information

Updated the test suite & Readme.md