Math module extended: Quaternion interpolation.

Quaternion interpolation, both linear and spherical, is now introduced.
Some types conversion warnings were removed.
CoreEngine instance in the GameNode.
LookAtComponent added.

LookAtComponent allows the game node to look at the camera all the time.
It can easily simulate a character talking with the player.
Minor change in TestGame.
LookAtComponent uses transformed position.

LookAtComponent uses transformed position of the camera. This way the
global position of the camera is taken into account instead of the
relative position of the camera to its parent game node.