Add .gitignore

This is the standard GitHub .gitignore file for new
Objective-C projects. This also removes the previously commited
xcuserdata files, which will just be recreated locally outside
of GitHub the next time the project is run.
Merge branch 'add-gitignore-for-xcuserdata' of https://github.com/dominic/10KTimer into dominic-add-gitignore-for-xcuserdata

Conflicts:
	10KTimer/10KTimer.xcodeproj/project.xcworkspace/xcuserdata/joy.xcuserdatad/UserInterfaceState.xcuserstate