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.
Also, you just called these `Item`s "Blocks" in your docstring above. Is that what they are? If so, why not just use that name?