Add support for external CSS stylesheets and marginally improved CSS parsing.

This pull request supports external CSS stylesheets by relative path. CSS rule parsing is also improved to support element and ID selectors in addition to class selectors, as well as combinations of selectors.

Note that if an external stylesheet isn't found at a relative path to the original SVG file it is then searched for in the bundle and then in the user's document directory. This allows NSString-sourced SVG to specify an external stylesheet, and also allows apps to write custom stylesheets at runtime which can be loaded by the parser.

SVGKImage is also improved to better support initWithCoder, allowing good behavior when SVGKImages are placed in NIB/XIB files.