Implement logging for environmental (non-entity) kills

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)

As this could be spammy (undead dying every morning, chicken murderdeathkill machines etc.) I'd appreciate a second (or third, or fourth....) opinion :)