Adds sandbox options to configure infectivity, infection range, and duration of infectious period.
Licenses the code under the Mozilla Public License 2.0 (MPL 2.0). I didn't license the translations file because I didn't think anyone would need to touch it, but if someone would like me to license it under MPL, I'll do so.

Implement basic functionality: corpses less than two days old cause infection with half the distance and threat of live zombies. No sandbox options yet.