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
Some more ideas for naming, also thinking ahead about the possibility of FluoClient having a newTransaction() method.

interface Snapshot 
interface CloseableSnapshot extends Closeable, SnapshotBase 
interface Transaction extends Snapshot  //used by Observer and Loader
interface CloseableTransaction extends Closeable,Transaction  //also has a commit() method