callbacks for leaving/entering proximity of an obstruction. hooray

Adds ability to set a `proximity` attribute on obstructions.

```coffee
  id: "table"
  src: "images/table.png"
  x: 260
  y: 400
  proximity:
    radius: 100 # within 100 pixels of the hit area
    onEnter: (item, options) ->
      console.log "ENTERING RADIUS"
    onLeave: ->
      console.log "LEAVING RADIUS"
```