Adds an emote handler and implements the /dance emote

[poffdeluxe]

Adds a basic emote handler that the chat handler can use. I decided to break the emote stuff into its own module since there are a lot of emotes and these emote definitions could probably grow really quickly but happy to roll all into the main chat.ex file if you think that's the best play from an organizational perspective.

Also, added a helper to easily get pids for players that are within a certain spatial range. Maybe an outstanding TODO here is to unify and abstract all the different player/entity access methods

Closes #12

[pikdum]

Made a few small tweaks:

• target guid wasn't packed, so needed to grab directly instead
• to save some memory, instead of adding mob names to :guid_name ask the entity process for the name
• add some more emotes to the mapping table