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.
The automated Travis Ci build should help us make sure that these programs run with all nightly version up to Rust 1.0 (and beyond?), so this comment should be removed (or we should update it at each rust nightly... :) )