NPC Updates, minor bomb & slime changes

- Changed slime hit boxes to not be so square
- Changed multi-pane dialogue to use arrays instead of strings (Thanks
Saki!)
- Changed NPC creation function to take an object. I am now passing
object literals to the function to create NPCs.
- NPCs can now have a dialogue 'callback'—a series of script to
evaluate once conversation has concluded.
- This dialogue callback can occur once or every time, on an NPC by NPC
basis.
- "Shy Kid" (in the bushes) now says one additional thing the first
time you talk to him (one-off callback).
- New NPC "Healer" heals you after speaking to you (eternal callback).
- Removed starting bombs.
- Added chest to secret area that gives you 5 bombs, and adds them to
your inventory so you can select them for use.
- Enemies don't drop bomb pickups unless you have the "Bombs" secondary
inventory item.