Add support to prerender service

[mrbatista]

This PR add support to prerender service and allows single page web application to be crawled perfectly by search engines.
This is a first implementation, improvement are welcome!
Configuration of static.json file:

  "prerender": {
    "token": "your_secret_key",
    "server_name": "your_server_name"
  }

• Add unit test
• Update readme with configuration

[edmorley]

Hi

This buildpack is now deprecated and we are recommending people move the more actively maintained heroku-buildpack-nginx. For migration advice see here.

As such, I'm closing this PR out since we won't be making further changes to this buildpack.