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.
@pfalabella Great points. Regarding point 2 - doesn't the `read_line()` return a `String` instead of `&str`? For example the following gives an error - 

```rust
let expr: &str = reader.read_line().ok().expect("Failed to read line!");
```