Documentation clarification on the gravity property
Cleaned some Audio init code, and added back a lost feature where the order of requested audio format was used as a priority (see #153)
Updated the list of contributors (drop me an email if your name is missing)
Fixed a limitation that was preventing a non visible layer to be set visible later

note that this is not applicable to collision layer, which are force to
not visible unless the `me.debug.renderCollisionMap` is set.
Revert "Fixed a limitation that was preventing a non visible layer to be set visible later"

This reverts commit abc73021e27328b99dc8a49f3b670940b9d84b9d.
Fixed a limitation that was preventing a non visible layer to be set visible later

note that this is not applicable to collision layer, which are forced to
non visible unless the `me.debug.renderCollisionMap` flag is set