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.
Might be useful to actually make Item an explicit class, and refactor this predicate into an actual method on that class. (Could you summarize this predicate into a single descriptive name?)