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'd rather not include such convenience macros at this time. This sort of stuff should be covered by the new `OsStr` types coming in the Rust IO reform. https://github.com/rust-lang/rfcs/pull/517