don't add margins if computedWidth of slide is zero

If an img hasn't loaded yet, outerWidth() will be zero. The marginLeft in that case would be half the container width. This breaks layout pretty badly when the image loads after adjustSlide() has already run. Better to not apply margins in this case, and listen for img load in another way.
Merge branch 'master' of https://github.com/baloneysandwiches/cycle2 into baloneysandwiches-master
version bump