Add a thread safe mutable dict class

Useful for the objectDictionary in QSLib
Merge master into threadsafe branch
Add copyright/credits
Minor touchups to the threadsafe mdict class;
Remove @synchronized calls for objectDictionary

We're using an OSSpinLock now instead
Merge pull request #1794 from quicksilver/threadsafe-objdict

Add a thread safe mutable dict class