Added checkRoute function to check if a route is defined into router

This plugin is awesome, congratulations for your work!

I have added a checkRoute function to be able to know which routes are defined in the router. I've used it to automatically differentiate between inner and external URLs in order to watch any link click in the page and route it accordingly. Also edited the README file to show this change and my editor removed some whitespace.

I don't know if it exist a better solution for this, but this is my code to listen for user clicks by now:

    $('a').click(function(e) {
      var url;
      url = $(this).attr('href');
      if ($.router.checkRoute(url)) {
        $.router.go(url);
        e.preventDefault();
        return false;
      }
    });

Maybe something like this could be activated by default or with an option inside of `bindStateEvents` function so page will start responding to any internal URLs with the router automatically, what do you think?

Best regards from Canary Islands! :-)