Update .gitignore

https://github.com/github/gitignore/blob/e3fd0442686165da9d6a11534988d0ec1329537b/Objective-C.gitignore
Enable Automatic Reference Counting (64-bit only)
Replace ivars with properties
Automatic build settings update
Link frameworks automatically
Modern import syntax
Delete unused build phase
Generalize build settings
Additional build settings cleanup
Remove localizations
Add MIT License
Stylistic changes
Remove unnecessary code
Convert tabs to spaces