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
was looking at curator code, this will throw ISE if takes more than 10 secs.  In this case when timer thread calls this function, do we want timer to die or keep going?