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 can leave it as `fn is_operator(op: &str)` but change the place where you call it from `if is_operator(&token)` to `if is_operator(token)`:

reduced example: http://is.gd/at1SGk