adds gitignore for Tower

Hi,

I added a gitignore file for Tower.app and puppet-librarian.

Tower.app is a OSX GUI for Git (http://git-tower.com/) allows "styling" of Git repos by including a logo / image called "Icon.png" in the roof of the repo. 

puppet-librarian is a module management system for Puppet. Read more at  https://github.com/rodjek/librarian-puppet