Created basic zombie that can path using PolyNav.

A lot of cleaning up needed, though.

I have to create different scripts to avoid breaking code. What remains
now is to modify all the other scripts (behaviours, zombie resources,
etc.) to make use of PolyNav, and discard the old scripts.

Also, I have removed the melee weapon from the navZombie. I find that
although it makes for a nice attack feel, it does us no favours in
making our programming easier. The code is still there nonetheless.