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
Could possibly have two locks, one lock for updates to zk node.  Another for updates to timestamp set.   Also, could have the concept of an advertised status and current status.   Maybe alwys compute current status based on size of timestamp set instead of storing it.