title: 3d geometry: how to align an object to a vector
body: <p>i have an object in 3d space that i want to align according to a vector. 
  i already got the Y-rotation out by doing an atan2 on the x and z component of the vector. but i would also like to have an X-rotation to make the object look downwards or upwards.</p>
  
  <p>imagine a plane that does it's pitch yaw roll, just without the roll.</p>
  
  <p>i am using openGL to set the rotations so i will need an Y-angle and an X-angle.</p>