Only preserve bounds if there is no shearing in the transformation.

Closes #358.
Clean up Matrix documentation.
Implement Matrix#skew()
Implement Item#skew() and clean up Item transformation functions documentation.
Use new Matrix#skew() in SVGImport.
Fix decomposed transformation sequence.
Implement Matrix#skew() calls for SVG skewX / skewY correctly.
Add missing break statement.
Only decompose matrix if there are bounds to be reused.
Merge git://github.com/paperjs/paper.js