Adding aspect ratio support for ZoomTool's box selection mode

Per the user request on the enthought-dev mailing list:
https://mail.enthought.com/pipermail/enthought-dev/2013-January/031979.html

It's easy to test this by modifying almost any of the examples that use ZoomTool, and just adding e.g. "aspect_ratio=1.5" to the ZoomTool constructor.  Tested with positive, negative, and zero aspect ratio.