fabric.Group objects toSVG() order changed

Changed the sort order in toSVG() from "first..last" to "last..first".
If they rendern on canvas the same order is used https://github.com/kangax/fabric.js/blob/master/src/group.class.js#L243
Merge pull request #425 from Kienz/patch-1

Change order of fabric.Group objects toSVG()