Heroku support

To make it easier for others to user JSBin hosted on Heroku, I have included all necessary changes to support hosting on Heroku (without any documentation though).

The changes can be summarised as:

- Procfile to launch correct process
- Support for a HOST field so that the correct URL is used for assets
- Support for standard PostgreSQL database
- Support for API set up with API_ALLOW_ANONYMOUS and API_REQUIRE_SSL environment variables.