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
having close() in Transaction in API is weird, since commit() is not currently in API.  Think the hierarchy needs to change.  Maybe something like.

interface  SnapshotBase  //has all methods for reading data (not sure about name)
interface Snapshot implements Closeable, SnapshotBase  //this is what FluoClient returns
interface Transaction extends SnapshotBase