Correctly fit/center SVG with negative offsets.

In my case, I have SVG with elements at negative y. I found that
the 'center' option actually makes them invisible - the element
on the top is the one with zero y coordinate.

It appears the code now uses getBoundingClientRect, not getBBox.
Here's what I get on my example:

	boundingClientRect: ClientRect
	height: 667.8125
	top: 55.1875
	...

	bbox: SVGRect
	height: 667.8125
	y: -17.8125

That 'top' is both ignored right now, and is in HTML coordinates -
the top of SVG is 72px, 72-17.8125 is exactly 55.1875. I presume
it's possible to use getBoundingClientRect, computing 'y' using
top as well as svg.offsetTop, but using getBBox() seems more
straightforward.
Merge branch 'master' of https://github.com/vprus/svg-pan-zoom into vp
stop zooming if outside specified zoom limits
fixed bug where controls were covered up by svg content in IE.
update getBoundingClientRect to make behavior consistent between Firefox and the rest.
Merge branch 'master' into gh-pages