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.
From the documentation: "It is recommended that only one handle at a time is created for the stdin of a process."

This means it would be better to use `let mut reader = io::stdin();` (before the loop) and then use `reader.read_line()` where necessary.