Serve the app with a devserver

[alexeagle]

To try it:

./node_modules/.bin/ibazel run :devserver
# prints Server listening on http://myhost:5432/
# open http://myhost:5432/src in browser

TODO before landing:

• make livereload work, /cc @gregmagolan
• get an rxjs release with the bits we need rather than using Greg's fork
• don't require me to go to /src in the browser, serve it at root