Design doc update

I updated the text of the design document in some places to bring it a bit more in line with our current design.  Check out the changelog in it for details.  The biggest thing is probably the addition of our current level to the level design section.

The software architecture diagram is out-of-date, but to be honest I don't really care.  Some of the other pictures might be a bit out-of-date also.

Anyway, take a look.  I'll merge in a little while and tag the repository if we don't have any problems with this.