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.

source/blender/python/api2_2x/Bone.c

@@ -69,7 +69,7 @@ static double boneRoll_ToArmatureSpace(struct Bone *bone)
 	Mat3Inv(imat, postmat);					
 	Mat3MulMat3(difmat, imat, premat);	
 
-	roll = atan(difmat[2][0] / difmat[2][2]); 
+	roll = atan2(difmat[2][0], difmat[2][2]); 
 	if (difmat[0][0] < 0.0){
 		roll += M_PI;
 	}