function setAxisAngle(out, axis, rad) {
  rad = rad * 0.5;
  var s = Math.sin(rad);
  out[0] = s * axis[0];
  out[1] = s * axis[1];
  out[2] = s * axis[2];
  out[3] = Math.cos(rad);
  return out;
}
Sets a quat from the given angle and rotation axis,
then returns it.

@param {quat} out the receiving quaternion
@param {vec3} axis the axis around which to rotate
@param {Number} rad the angle in radians
@returns {quat} out