Add deploy-to-heroku button

I have added the **app.json** file and modified **README.md** to support the
deploy to heroku feature.

All parameters from documentation have been inserted into json file.
Note: looks like
    
        app = require('../app')

will prefer _app.json_ over _app.coffee_. For this reason, I had to specify
extension for the app.coffee file.

Please have a look at comments too.
V.