Add heroku shell command to the fabric file.

The shell can be useful to inspect models and execute
some commands.
Add instructions for deploying to heroku.

https://github.com/madewithbytes/us_ignite/issues/39

The application has been deployed to heroku
http://us-ignite.herokuapp.com/ and mechanics
have been provided to do so.

The prerequisites have been documented in the
README.rst file.

Closes #39.