First Batch of fixes for #4

[Judgy53]

Everything should now be scaled on graph size (except zoom max level and margin) and zooming should feel a bit smoother. There are still a few problems :

• The "double scrollbar" feel but that's more of a global issue.
• Panning doesn't exactly follow the cursor. That's because mouse events gives us a screen position, which are unusable as such on a viewboxed svg. There is a way in JS to deal with this case, but I didn't find it's equivalent in elm. Was thinking about using ports but I would like your feedback on that.
• The clamping is off by a few pixels on the vertical axis and I have no clue why. The added margin mostly "fixes" it but it's still bothering me. Help needed here.

[erosson]

The viewbox could probably just go away -

Implemented this in d67f529352ed40cd0ea640fd242d13d95c1ddcfd / #4. You wrote most of the zooming/panning, so after you take a look, I'm feeling ready to release it (without ?enableZoom=1, I mean).

[Judgy53]

Looks really good to me, the max zoom is maybe a bit too much, but it doesn't really matter.