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.
@aochagavia with a `Return<(), String>` in case of success he would return `Ok(())`, in case of error he would return e.g. `Err(format!("Wrong answer. Result: {}", x))`.
Bu maybe I didn't get what you meant?