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.
Tried exactly the same thing as the example above Paolo. I'm getting a `lifetime mismatch` error

```rust
src/24_game_rpn.rs:80:35: 80:38 error: mismatched types: expected `&&str`, found `&&str` (lifetime mismatch)
src/24_game_rpn.rs:80     ["*", "-", "+", "/"].contains(&op)
                                                        ^~~
src/24_game_rpn.rs:79:34: 81:2 note: the anonymous lifetime #1 defined on the block at 79:33...
src/24_game_rpn.rs:79 fn is_operator(op: &str) -> bool {
src/24_game_rpn.rs:80     ["*", "-", "+", "/"].contains(&op)
src/24_game_rpn.rs:81 }
note: ...does not necessarily outlive the static lifetime
error: aborting due to previous error
```