Fix graphene_quaternion_init_from_angles calculations

This was just wrong (did not even set z!)
This implementation is the same as the one in three.js:

https://github.com/mrdoob/three.js/blob/master/src/math/Quaternion.js#L119
Merge pull request #15 from alexlarsson/master

Fix graphene_quaternion_init_from_angles calculations