First draft of typesetting engine

I implemented the Knuth-Plass algorithm. If necessary, I can simplify the code (the current version may work for circle or triangle-shaped text, [see this example](http://www.bramstein.com/projects/typeset/) but we don't need such advanced use).

The PostScript part still needs to be done.
Haha! (I'm assuming this is going to become a `raise AssertionError` or something similar? (`assert first_active_node` would suffice I guess.)