Use Rerun gem for auto code reloading in development/test

I've been using [rerun](https://github.com/alexch/rerun) in my local wd-sinatra app for auto code reloading. I know in the README, you said you wanted to leave it up to the developer to implement, but I think we should have a baked in solution so that the end user can get up and running quickly developing.

I chose rerun, because:

1. The Sinatra FAQ [recommends it](http://www.sinatrarb.com/faq.html#reloading)
2. It uses [listen](https://github.com/guard/listen) so it can support different OS platforms
3. It should work with any ruby application server since it'll just restart the process
4. All it requires is adding rerun and the notification gem for your platform to enable it, no other configuration.

All you do is:

    $ bundle exec rerun -- rackup