Correct README to show how to require deque, rather than queue
Removed Hamster::Stack

Hamster::Deque is 100% as efficient when used as a stack as Hamster::Stack is.
The documentation has been amended to draw attention to the fact that a Deque
can be used as a stack.

Closes #132 and #57.
Added Wikipedia link to Deque docs
Annotated EmptyTrie as @private