Add GitHub's official Objective-C .gitignore file.

Hi Eugene,

One of the steps from Lab 11 was to add the official GitHub Objective-C .gitignore file. I'm submitting this to you as a Pull Request. You can accept it now, or wait for help in class.

This will remove files from your project that you really don't want
in git. For example the .DS_Store files and xcuserdata stuff.