[#546] fixed collision layer rendering issue with isometric map
avoid referring to `_default` when calling `sound.play()`

this is an internal value/reference only, that changed anyway in the 2.0
branch. @see discussion here :
https://github.com/goldfire/howler.js/issues/193
[documentation] fixed the `me.body.onCollision` parameter type and description
[#543] removed the old useless object types under `me.game` (replaced by me.collision.types)
fix a link in the README file