Add Heroku support, and other bits

- Make sure uberjar builds work and contain all the Clojurescript
- Give {{name}}.server a -main function so we can run straight from the jar
- Add a Procfile, and a system.properties so we get a recent Java
- Add a sane .gitignore so a `git add -A` does the right thing
- Disable REPL and Figwheel when running in production. This part is a
  bit hackish, feedback welcome.
- Significantly update and expand both README's (Chestnut's, and the one
  you get with your app.)