Support keyTimes and keyPoints in Motion path

keyTimes is mentioned in
http://www.w3.org/TR/SVG/animate.html#KeyPointsAttribute
and defined in
http://www.w3.org/TR/SVG/animate.html#KeyTimesAttribute

keyPoints is defined in
http://www.w3.org/TR/SVG/animate.html#KeyPointsAttribute
These values may be equal. You'll probably want to make sure they're not in _findLeftIndex() to avoid divide by 0.