Implement logging for environmental (non-entity) kills, such as suffocation.  Adds #80

This change adds a new config option, logEnvironmentalKills.  When set to true,
Logblock will record entity deaths not directly caused by another entity.
(for example, suffocation, or burning to death)
Merge pull request #499 from frymaster/nonentity-kill-logging

Implement logging for environmental (non-entity) kills