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
Nevermind- scratch this. Operations.initialize() takes an Accumulo Connector so the Zookeepers will always be the same. This would also mean bad zookeepers on the Accumulo side would affect its ability to schedule the compaction. I should have checked before i wrote the previous comment.