Allow all rotation angles

Fixes #194.

I hope my viewport calculations are correct.

I also have code which allows for pinch rotate, would you like me to create a PR for that too? If so, should I add an option for it?
You haven't actually changed this function to work with non-right-angle rotations, so we can't remove this check. In fact, there's no way to write a non-right-angle rectangle rotation function that returns a Rect, since x/y/width/height is not enough to encode a rotated rectangle. 

Looking around the code, it looks like the only place we'll need to address a non-right angle usage of Rect.rotate is:

https://github.com/openseadragon/openseadragon/blob/master/src/navigator.js#L244

Perhaps that can be rewritten to not need Rect.rotate?