use pg gem in production environment
use thin (version 1.5.1)
add Procfile for foreman (on Heroku)
specify Ruby version for Heroku

ref. http://kyanagimoto.blogspot.jp/2013/05/rails400-rc1-herokudeployactiveresource.html
avoid routing error /assets/application-****

ref. http://stackoverflow.com/questions/7829480/no-route-matches-get-assets
Merge branch 'heroku'
fix items controller test