add .xccheckout to Objective-C gitignore

This is a file that Xcode 5 uses for version control, doesn't need to be checked in. Also see http://stackoverflow.com/a/18448100/1877078
Merge pull request #774 from bdewater/patch-1

add .xccheckout to Objective-C gitignore
Merge remote-tracking branch 'upstream/master'