Makes object identity checking threadsafe, closes #45

Additionally included is a thread safety test which we unfortunately can't make use of at the moment due to the fact that Scala reflection is currently not thread safe on Scala 2.10.x. 

Fortunately, we're only a week or so away from thread safe reflection fully merged into scala/master. This means that folks running scala/pickling on 2.11 milestones beginning with 2.11-M7 will not have this limitation.