updated the `playTrack` function to returns the current sound instance (as the `play` function is doing)
Merge branch 'master' of https://github.com/melonjs/melonJS
Fixes sync issue
[#470] updated documentation and added instance_id parameter to stop/pause functions
Sync'ed with master branch
reorganized source code for all basic object definition/augmentation
[#291] Disable ObjectEntity falling flag when there is no gravity.

- Thanks @Deathspike for #478
[#482] Fix an exception in ObjectEntity.checkCollision()

- See: https://groups.google.com/d/msg/melonjs/w6UawD-I3Dw/SNY1cNy8wcEJ
- Reset the me.game.collisionMap reference to an empty collision map.
- The empty collision map is only created once at startup.
Update CHANGELOG