add gitignore file

![screen shot 2014-06-16 at 2 51 08 pm](https://cloud.githubusercontent.com/assets/216584/3291717/500975d8-f587-11e3-8ba5-80ea723b775a.png)

I add a gitignore file to avoid having problems (like this one) when I use IBCustomFonts github repo as a submodule for my projects.