Update the Git Ignore file, to ignore auto-generated project settings files.

References:
- http://kleber-swf.com/the-definitive-gitignore-for-unity-projects/
- http://www.strichnet.com/using-git-with-3d-games/
- http://stackoverflow.com/questions/18225126/how-to-use-git-for-unity3d-source-control
- http://blog.i-evaluation.com/2013/09/12/unity3d-with-git-gitignore-for-bitbucket-and-github/