Major changes to the generation loop.

Moved most of the loop to the Dungeon class. This will address several
issues. Issue #147

* Regen all now works. Closes #105

* Dungeons are saved more often. Issue #157

* maximize_dist now takes old dungeons into account. Issue #157

* Better dungeon packing. Dungeons are now scaled down in X, Z, and level instead of just X, and Z, THEN level.

* Changelog updates