immutable some things
first pass at some obvious thread safety problems
more thead safety fixes
import OSAtomic
Merge branch 'master' into thread-safe

Conflicts:
	Classes/GTObjectDatabase.m
more thread safety
don't actually need that
more asserts, less laziness
you get a strong ref to repository, and *you* get a strong ref to repository
remove unnecessary atomicity
call through to the other initializer
specta-fied some tests and removed unnecessary tests
Merge pull request #188 from libgit2/thread-safe

Thread safety