Optimized for deploy/run to Heroku

- Added some gems to Gemgile, and updated Gemfile.lock
  - PostgreSQL in production
  - rails_12factor gem
  - Unicorn
- Defined web proccess with Procfile
Added Heroku Button

- Put Heroku Button on README.md
- Added app.json
- Added config/secrets.yml for read secret_key_base from env in priduction
Added config/initializers/assets.rb
Merge pull request #179 from ayumin/heroku-button

Heroku Button