Generator should not mutate the PriorityQueue

`extension PriorityQueue: GeneratorType {
    public typealias Element = T
    public func next() -> Element? {
        return pop()
    }
}`

Just iterating the elements causes the PriorityQueue to be cleared out. 
If you use any of the swift functions that iterate over SequenceType, functions like `contains()` the queue also gets cleared.
