Implement `input_method` artifact

This artifact will link the input method to `~/Library/Input Methods`.

I have also attached a cask(`openvanilla.rb`) using the new artifact.

After installing the new cask, user can add the new input method in Input Sources inside Keyboard prefpane.
 
![screen shot 2014-01-14 at 1 24 23 am](https://f.cloud.github.com/assets/667272/1902824/064e8de4-7c78-11e3-94ad-5fb3a58ba7f8.png)

Thanks @rolandwalker helping me out on the new Symlinked class and debugging the `input_method` artifact.

P.S. Input Method is different from Input Manager.