Math: handle exceptions on 0 values + simplification.

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Rendering : ugly mode

Merging moto rendering of ghost/rider.
Rendering from anchors, not from positions.

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Ghost : replaying the ghost based on frame time, not frame count

Adding a frame time to frames.
It allows to save frames to different framerate.
It looks like a bug if you load the game, make a ghost a replay the level : during the 1st second, the ghost is doing nohting. In fact, the problem is that the start_time of the level is not when the page is finished to be loaded.
Level start time should be fixed to the end loading time.

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Timer : add cents

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Physics : add a head to the rider

Add a physic head to the rider, and thus, reduce the torso size.
Review listeners for collisions
Change ghost color

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Minor fixes

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>
Merge branch 'master' of https://github.com/MichaelHoste/xmoto into replays

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>

Conflicts:
	bin/xmoto.js
	index.html
	src/constants.coffee
	src/level.coffee
	src/moto/ghost.coffee
	src/moto/moto.coffee
	src/moto/replay.coffee
	src/moto/rider.coffee
	src/utils/assets.coffee