Add MessageBox functions and string conversion macros

Usage example:

```rust
#![feature(phase)]
#[phase(plugin)] extern crate winapi;
extern crate winapi;
use std::ptr;

fn main() {
    let text = "Lorem ipsum dolor sit amet, consectetur adipisicing elit";
    let r = unsafe{ winapi::MessageBoxW(ptr::null_mut(), _W!(text), _W!("MessageBox testing"), winapi::MB_ICONWARNING|winapi::MB_OKCANCEL) };
}

```
I'm using the strict definitions of `HWND` from the Windows headers. This means that `HWND` is a unique type. If you want to easily convert between `HANDLE` and `HWND` you'll have to do a transmute.