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.
This looks much better IMO. However, to make this work I need to pass in a pointer to &str otherwise the compiler complains 
```
error: mismatched types: expected `&&str`, found `&&str` (lifetime mismatch)
src/24_game_rpn.rs:85     ["*", "-", "+", "/"].contains(&op)
```
The following works, although I'm not sure if this is a good practice. What do you suggest?
```rust
fn is_operator(op: &&str) -> bool {
    ["*", "-", "+", "/"].contains(op)
}
```