Box<T: Error> should implement Error (std::error::Error)

I'm not too sure what versioning-annotations to use, otherwise I'd do a PR myself.

``` rust
impl<T: Error + ?Sized> Error for Box<T> {
    fn description(&self) -> &str {
        Error::description(&**self) 
    }
    fn cause(&self) -> Option<&Error> {
        Error::cause(&**self) 
    }
}
```
