Added RPN version of 24 game

An alternate implementation of the 24 game problem which uses RPN (reverse polish notation expressions). 

FYI [RosettaCode](http://rosettacode.org/wiki/Reports:Tasks_not_implemented_in_Rust) still shows that 24 game and 24 game solve are not yet completed.
you could also express this as `["*", "-", "+", "/"].contains(&op)` but it's a matter of taste (the way you wrote it might actually be clearer)