fix($animate): ensure CSS driver properly works with SVG elements

The default CSS driver in ngAnimate directly uses node.className when reading
the CSS class string on the given element. While this works fine with standard
HTML DOM elements, SVG elements have their own DOM property. By switching to use
node.getAttribute, ngAnimate can extract the element's className value without
throwing an exception.

When using jQuery over jqLite, ngAnimate will not properly handle SVG elements
for an animation. This is because jQuery doesn't process SVG elements within it's
DOM operation code by default. To get this to work, simply include the jquery.svg.js
JavaScript file into your application.

Closes #6030