rename all 'skew' methods to 'shear' and implement real skew

your `skew` methods are actually `shear` methods - similar, but not the same.

http://upload.wikimedia.org/wikipedia/commons/2/2c/2D_affine_transformation_matrix.svg

skew takes an angle, shear takes an amount. i renamed all the skew methods to shear and created actual skew methods that are compatible with css transforms. i ran into the distinction while implementing & testing my `decompose` branch: https://github.com/leeoniya/transformation-matrix-js/tree/decompose