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

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`.

Note that jQuery still uses `className`, so developers who include jQuery will have this functionality broken. The solution for them is to have jQuery fix it. :)

`classList` was also considered as a solution, but because only IE10+ supports it, we have to wait.

Closes #3858