Masks of Nar'sie - Testing Version

So I got this working! Hurray!

This version of Masks of Nar'Sie adds the Mask entity to the Cult game-mode and extracts and modifies how AI vision works. 

I'm making this pull request because I think we need to test this idea on the server before going any further with it. While I think the Mask Entity concept should be a useful addition to the Cult game mode, it has the potential to be hugely disruptive and annoying. 

It would make sense to test this minimal feature version in full-population rounds to see if the idea has merit.

MAJOR CHANGES

- extracted the AI's selective vision system into an abstract class
- re-instantiated the abstract vision system for the AI
- created the Spirit mob type
- created the Mask Of Nar'sie mob type
- added a number of verbs to cultists to interact with the Mask
- re-factored the rune code to be more readable and searchable
- modified the way in which cultist icons are displayed