Add support for a HTML 5 History API fallback

I had some good fun tonight running the dev server. Unfortunately I couldn't find a way to setup it so it serves the index.html page when a non-existing page is requested. The feature is quite handy in development of single page applications (SPAs) and the otherwise splendid did server not-supporting it has kind of ruined all the fun.

So, the PR implements this: if you pass the `--history-api-fallback` option to cli, or pass the `historyApiFallback: true` option in the Node API options, the server will then serve /index.html on any requests that don't happen to have another match.

This is implemented with the help of the [connect-history-api-fallback](https://github.com/bripkens/connect-history-api-fallback) Connect middleware. I've used this in some other places and it works perfectly.

I've tried to update the README and the examples to match the new feature. Please let me know if there's anything I have missed.

I hope this will be merged in someday. :)