Abstracted Overlays and the start of a bank

Added overlays, which include:
 ~A number variable in the hero class that describe what, if any,
overlay that hero is looking at.
~ An extra if statement to hero's update to check if there's an
overlay. If there is, then nothing expect jumping works atm, which
closes the overlay.
~A bank class that changes the hero's overlay value when interacted
with.
~drawOverlay(hero) in viewport, which has a switch statement to draw
the appropriate overlay.
Merge branch 'master' of https://github.com/knarr/winter

Conflicts:
	client/stage.dart