Added a, b, c, d, tx, ty setters to Matrix 

Hi Bernhard,
Would be great if you could merge, as I would like to have [dart-commons](https://github.com/blockforest/dart-commons), [spring-dart](https://github.com/blockforest/spring-dart), [rockdot-dart](https://github.com/blockforest/rockdot-dart) and awesome examples (soon!) rely on the "official" StageXL, not some fork of mine. 
regards Nils