fix issue #3 "Cannot call method 'toFixed' of undefined" on 90deg rotated faces

As mentioned [here](https://github.com/thomasxiii/photon/issues/3), the error "Cannot call method 'toFixed' of undefined" appears in some case when faces are rotated to 90deg.

To be honest, I don't know what was missing exactly, but it seemed that the problem was related to the CSSMatrix.
So I tried adding the full FirminCSSMatrix class code from [here](https://github.com/beastaugh/firmin/blob/master/src/matrix.js), and it solved the problem.

Photon.js without changes :

http://jsfiddle.net/JulianG/X7FZA/

Photon.js with full FirminCSSMatrix class :

http://jsfiddle.net/JulianG/yS8sF/