Added NPCs to Level - render and update

Level now creates and holds onto/updates the NPCs. NPCs are stores in an
ArrayList for access and also in a HashMap with coordinate keys for
rendering.
Level constructors "addNPCs" to your ArrayList
update updates your NPCs and also the HashMap
Render renders your NPCs
Fixed a grammatical error in a question about grammar

I was happy to find that my HashMap<Integer, ArrayList<NPC>> didn't slow
the game down to a crawl. Doesn't look like it affects the FPS at all
since having 0 and 1000 NPCs gives me the same framerate!