Add autocompletion for Rails3.

+ Originally published by Christopher Chow.
+ Source:
https://github.com/robbyrussell/oh-my-zsh/blob/30620d463850c17f86e7a56fbf6a8b5e793a4e07/plugins/rails3/_rails3
  Commit: 30620d463850c17f86e7a56fbf6a8b5e793a4e07
Merge pull request #1764 from johnjohndoe/feature/rails3-autocompletion

Add autocompletion for Rails3.