Deprecate GTMObjectSingleton for 10.9/iOS 7 to force removal of the dependency from projects using it.

DELTA=8  (7 added, 0 deleted, 1 changed)

git-svn-id: http://google-toolbox-for-mac.googlecode.com/svn/trunk@617 7dc7ac4e-7543-0410-b95c-c1676fc8e2a3