fix(jqLite): use get/setAttribute so that jqLite works on SVG nodes

jqLite previously used `elt.className` to add and remove classes from a DOM Node, but
because the className property is not writable on SVG elements, it doesn't work with
them. This patch replaces accesses to `className` with `get/setAttribute`.

`classList` was also considered as a solution, but because only IE10+ supports it, we
have to wait. :'(

Closes #3858