Improve scaling by making it dramatically simpler.

- Add test for scaling an SVG.
- Use viewBox for scaling SVG (see https://github.com/poptip/saveSvgAsPng/commit/a93874389411cb5b6db4f1e27832ab1321381469).
- Remove scaling group element.
- Remove moving of children from one parent node to another (hopefully helps with #5, #6, and #7).