Obsidian Menu Bar for 10.9 Mavericks cask

Followed https://github.com/phinze/homebrew-cask/blob/master/Casks/obsidian-menu-bar-mountain-lion.rb as an example. 

- Not sure if it's obvious or not to Log Out & Log In...
- Not sure if this is a good use of `caveats`
- Not sure if running the install script is good behavior

I'm new to contributing, please help me help you