Cascading style behavior for style elements.

When an element is defined in 'defs', style rules cascade into the defs
subtree.

Where a use element has inherited style rules, these are applied, but if
cascaded style rules apply at the point where the used element is
defined, those rules take priority over the inherited rules.
Add links to the SVG spec that explain how CSS Styles are applied.
Merge pull request #23 from ewilligers/cascaded-style

Cascading style behavior for style elements.