Make buildpack an nginx compiling heroku app.

* Add Gemfile & Gemfile.lock. This ensures the buildpack is detected as
  a ruby app when deployed to heroku.
* Add Procfile that automatically runs the compile script on deploy.
* The compile script:
  * Starts a ruby server instead of the python one.
  * Creates tarballs to make downloading the binary easier.
  * Indefinitely waits when compilation is done, to prevent the heroku
    instance from crashing.
  * Is documented better.
* Document the new way to build binaries in README.md