Support defaulting user-specific Xcode project settings.

Objective-C.gitignore

 # xcode
 build/*
 *.pbxuser
+!default.pbxuser
 *.mode1v3
+!default.mode1v3
+*.mode2v3
+!default.mode2v3
 *.perspectivev3
+!default.perspectivev3
 *.xcworkspace
+!default.xcworkspace
 xcuserdata
 # osx
 .DS_Store