Update dependencies necessary for Heroku.

Specify the ruby version within the Gemfile so it loads ruby-2.0.0
rather than 1.9.3.
Add unicorn web server.

Unicorn is the web server recommended by heroku. Also include the
Procfile for use with foreman.
Load secret tokens from environment.

Rather than hard-coding the secrets within the app load them from
an environment file. With this setup there can be different tokens
generated for the development and production environments.

The .env.sample file contains some example tokens that can be used
for development purposes.
Merge pull request #3 from radavis/heroku_deploy

Heroku deploy