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
This class doesn't seem to be Accumulo specific. Would it make sense to keep the zookeeper portion of the API using Curator like everything else and move this to core?