Temporary hack fix to prevent crash correlated to race condition between addObserver on low priority thread followed by immediate removeObserver on main (from @epatey)
Failing unit test to expose thread-unsafe implementation of -[RACSerialDisposable disposable].
Reimplement RACSerialDisposable with @synchronize rather than OSAtomic in order to fix -disposable method to be thread safe.
Merge branch 'serial' into talko_master