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 that as well. Still compilation error (lifetime mismatch). Here's how the `in_operator` looks

```rust
fn is_operator(op: &str) -> bool {
    ["*", "-", "+", "/"].contains(&op)
}
```