Act as a wrapper to Lasem

[Lasem](https://wiki.gnome.org/action/show/Projects/Lasem?action=show&redirect=Lasem) is a fast and easy math TeX -> SVG converter. Mathematical will now act as a wrapper around it. Preliminary tests show documents with over 800 equations rendering in less than three seconds.

Things to do before this merge:

- [x] work on base64 converting
- [x] catch against errors in tests (passing wrong values as options, for instance)
- [x] actually allow for option passing (dpi/zoom/forbid-markup)
- [ ] try not to depend on writing to a file