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
checking the truncation time may be out of scope for this issue... I am thinking the GC iter needs a comprehensive unit test.  I will open an issue for that.  That unit test could cover this case.