Need a shopkeep entity that will spawn in shops.

Shopkeep needs:

• GUI for trading/selling things
• AI to keep him in the front part of the shop if no zombies around.
• POSSIBLE: Price fluctuation based on civ rep level

We need to discuss what the shopkeep can sell and for how much. I would think we don't want to completely nullify villager trading, so maybe we should consider no/very little overlap with what villagers trade for.

Perhaps they'll just sell the items listed in issue #14