Evaluate Reverse Polish Notation

	/**
	 * Evaluate the value of an arithmetic expression in Reverse Polish
	 * Notation.
	 * 
	 * Valid operators are +, -, *, /. Each operand may be an integer or
another
	 * expression.
	 * 
	 * Some examples: 
	 * 
	 * ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
	 *  
	 * ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
	 * 
	 * */