Blender Git Loki

Fix for bug #5250: inaccurate conversion between edit and pose mode bones.

Using acos(dot(u, v)) to find the angle between two vectors is quite
inaccurate, and there's a better way to do it, as explained here:
http://www.plunk.org/~hatch/rightway.php

Also changed the use of atan for computing roll to atan2 in some places,
the latter avoids accuracy and division by zero issues.