Allow both intrinsic sizing and scaling to the parent’s width.

As discussed in #9, have both width/height and viewBox attributes.

As far as I understand, setting CSS 'height' to a percentage
(even though that resolves to 'auto') triggers this part of the SVG spec,
so that the used height is not just the "height" attribute.

http://www.w3.org/TR/SVG/coords.html#ViewportSpace
> if there are positioning properties specified on the referencing element
> or on the outermost svg element that are sufficient
> to establish the height of the viewport,
> then these positioning properties establish the viewport's height;
> otherwise, the ‘height’ attribute on the outermost svg element
> establishes the viewport's height.

Blink, Gecko and Presto seem to be consistent there,
but I don’t quite understand what makes the ratio be preserved.
In particular, any percentage value gives the same result.

Keep the part of #9 that fixes #3.