Add SvgImport example for Inkscape and fix issue with multiple SVG path commands.

Closes #203.
Define unit tests for Path#contains, some are currently failing.
Implement Curve#getCurvatureAt().
Fix issue with imprecise tangents / normals to curves at t = 0, 1.
Improve precision of Numerical.solveCubic() and fix issues in Curve.getCrossings().

Closes #202.
Remove tolerance parameter from solveCubic() / solveQuadratic() since we should always use EPSILON.
Remove cbrt() since we don't use it anymore.
Replace solveCubic() code with version that does not use cbrt() calls.
Optimize Shape constructors by passing along point.
Implement Item#hasFill().
Implement Item#contains() and use it for hit-testing Rasters.
Fix wrong Shape.Circle size.
Implement Shape#contains() and define unit tests for it.
Fix mistake in example code.
Rename 'properties' constructor parameter to 'object' in docs.
Add example to Item#contains(point) docs.
Add titles to Path.Constructors documentation examples.
Add Gradient class examples.
Improve GradientStop example code.
Improve Style example code.