How to get a signed angle ?

Hi,

I want to calculate the orientation of a `Vector2`. The sign of the angle matters, but `vector2.angle(Vector::x())` doesn't seem to keep that information.

example:

```rust
// assertion fails
assert_eq!(
na::Vector2::new(0f32, -1f32).angle(&na::Vector2::x()),
-std::f32::consts::FRAC_PI_2
);

```

Is there a way to do it with `nalgebra`, maybe with `Rotation` ? Or should I just do it by hand ?