Allow users to specify an aspect ratio for SquareStrategy

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.

Start by reading the existing SquareStrategy implementation and the discussion in issue #50. Define how a target aspect ratio should be specified, then determine whether RectangleStrategy should become the general implementation while SquareStrategy remains a compatible subclass; the naming and scope need resolution before implementation.

Written by the indexing model from the issue text.