Closes #8 - Made garbage collection iterator determine oldest active snapshot.  

Pull request for #8 

Created TimestampTracker to keep track of timestamps in Zookeeper.  Modified GC iterator to get oldest active timstamp from Zookeeper. Created integration tests for TimestampTracker and GCIterator
Do you know what java timer does when exception is thrown?  I am worried about info being dropped on floor.  Wondering if the run method should catch exception and log it.