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
An inner class can access outer class `this` by prefixing w/ class name.

```java
class Foo {
   class Bar {
       public void run(){
           synchronized(Foo.this){
           }
       }
  }
}
```