Enemies need AI

[tlhunter]

Enemies should have some path-finding capabilities and hunt down players.

[tlhunter]

There was a pull request to implement enemy AI. But now, wandering NPCs (e.g. NPCs not following a character) seem to bounce back and forth instead of moving randomly. Also, instead of walking straight towards a character, they zig zag.

One thing to think about is aggro radius: i.e. how close does a player need to be to an enemy for the AI algorithm to activate. Also weeping angels. https://www.youtube.com/watch?v=AabW5P8ft2g