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
nested enum types are implicitly static but I added ```static``` to enum so it's clear

http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.9