added two small fixes to svg.pyx

* Added logic to to stop svg.pyx from breaking upon seeing color="currentColor". "currentColor" is a valid [SVG color descriptor](http://www.w3.org/TR/SVG/color.html), but it's implementation is more of a CSS/web thing and I think it makes sense to consider it the same as "black" in kivy. This is the behaviour in inkscape.

* replaced direct array access (e[s]) with e.get(s) for lines, circles, ellipses.