New features for matrix classes
New features for matrix classes

Inverse of 2x2 and 3x3 matrices
Rotation setters and constructors for 2x2,3x3 and 4x4 matrices

Signed-off-by: John McCutchan <a51dda7c7ff50b61eaea0444371f4a6a9301e501@johnmccutchan.com>

julien/vector_math.dart,brason/vector_math,julien/vector_math.dart,brason/vector_math