Image in stacklayout, full width

I'm trying to scale an image (either up or down) which is in a stacklayout so that it's width is the full width of the screen. The height should be whatever height is required to maintain its aspect ratio.

What xaml markup is required to do this?

<StackLayout>
    <Label Text="Title"></Label>
    <Image Source="http://xxx"></Image>
</StackLayout>