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
We may want to capture some type of integration tests that fail with a bad zookeeper connection string but demonstrate Accumulo failing gracefully under the hood instead of allowing compactions to throw exceptions and getting rescheduled.