Replace MySQL with Postgres

* Postgres is the preferred db for Heroku (free hosting)
* Removed user/pass in `config/database.yml` file under development/test environments. Heroku will override production db settings so skipped them.

* How to set up on your machine:
http://railscasts.com/episodes/342-migrating-to-postgresql?view=asciicast
* Setup your database using

    rake db:create:all
    rake db:migrate db:test:prepare