Defining element.getComputedStyle for SVG items so that Highchart's anim...

...ation calls work with Fx.CSS.js

Calls to `.animate` in the MooTools adapter reference `Fx.Morph`. This in turn invokes methods in `Fx.CSS` including `Element::getComputedStyle`, a method not present on SVG elements. This pull adds this method to prevent errors from occurring.