Allow users to specify an aspect ratio for SquareStrategy

[pganssle]

Per my comments in #50, I think we should probably create a version of SquareStrategy that allows you to specify a target aspect ratio other than 1:1.

The nomenclature is all messed up and I'm wondering if we should rename this project and a bunch of the classes in it before too many people start using it, but for now we can just call it RectangleStrategy, and make SquareStrategy a subclass of it.