ADD: Stack.h.
A Template class Stack. It creates a Linked-List Stack. It has the following methods:
	void push(<element>) - pushes elements on top of the Stack.
	void pop() - if there is an element on top, it is removed; otherwise - an exception is thrown.
	<element> peek() - if there is an element on top, a copy of it is returned; otherwise - an exception is thrown.
	bool isEmpty() - returns whether the Stack is empty of not.