Added setUseShapeColor & getUseShapeColor to ofxSVG

Today the only way of overriding the color of an SVG is by looping over the internal ofShapes and calling setUseShapeColor on them from the outside. This seems a bit hacky & requires some knowledge about ofxSVG internals. I think it is better if this functionality is exposed through a pair of public methods.

Tested on Mac OS 10.7