Use the connect-history-api-fallback middleware to redirect 404s to /

This adds a simple connect middleware to the current server implementation to fallback SPA browser reloads to the root path.

I've skimmed through the currently open PRs and issues and it's not clear if it's ever planned to support this in Broccoli. Please let me know if it's a wrong way to go.

P.S. Broccoli rocks. :)