Add public method to clear all dataset requestCaches.

This pull request adds a public method ``clearRemoteCache`` to clear all remote requestCaches. If using a remote dataset, this is helpful when the remote data may have changed and you want to force typeahead.js to make an ajax call instead of using cached response data.

Note that this is not the same problem solved by https://github.com/twitter/typeahead.js/pull/476 or https://github.com/twitter/typeahead.js/pull/515. Those pull requests focus on clearing the dataset cache, whereas this allows a user to specifically clear all requestCaches.

If there already is a better way to do this, please let me know!