More matrix ops native functions

[Henrik Moller]

Added eigenvectors/eigenvalues, vector internal angle calculation, and 
identity matrix generation.

For those of you who might be into graphics, openGL rotation, instead of 
using decomposed Euler angles or Tait–Bryan angles, use a scalar angle 
and a rotation axis.  That's why mtx supports the mtx['a'] "angle" 
function and the mtx['c'] cross product function--the cross product of 
two vectors is the rotation axis around which you rotate to get from one 
vector to the other. (This kind of rotation specification makes it easy 
to do smooth rotations in animation.)