Don't parse JSON twice

The previous commit parsed the request body to JSON. Then attempted to run
`JSON.parse` on the `user` key of the returned hash. This doesn't work.
Update Gemfile

Replace the gemcutter source with rubygems.org and add `Gemfile.lock`.
Specify ruby version

Heroku runs Ruby 2.0.0 by default, so let's go there.
Use Postgres in production

Heroku doesn't work well with SQLite3 and the `data_mapper` SQLite backend
doesn't even install. The database is now configured through the
`DATABASE_URL` environment variable (Heroku standard name).
Re-add SQLite for development
Add Procfile

Useful for running locally and remotely. A sample `dev.env` has been added that
sets `DATABASE_URL` to `sqlite3::memory:`.

Also, move all development dependencies into the `development` group.
Update README and links

Link to new GitHub repo, refresh README with instructions for running on Heroku
and locally through Foreman.