Use Figaro gem for managing environment vars

Closes #15

**Gemfile**: added figaro gem
**config/application.yml**: added after running 'rails generate figaro:install', which auto-adds this file to .gitignore; includes github ID and Secret
**config/initializers/omniauth.rb**: using ENV hash instead of YAML load file call, this will allow ease of better setup for Heroku deployment with dev/test/prod client IDs and Secrets

#### References:
* Figaro Install run issue & solution: http://stackoverflow.com/questions/21274678/rails-could-not-find-generator-figaroinstall
* Figaro gem: https://github.com/laserlemon/figaro