Updated to current version 0.2.1

Changelog:

From now on, only one (static) die exists.
Power units can now have owners.
Properties are constructed in their own constructor, instead of in
Board.
Added purchase() and payRent() methods to Player.
A lot of code refactoring in various places.

This brings the programme up to date to Phase 3, *without* the bonus and
the extra test classes.