Fix svg element size

fixes #2506, closes #2517

This PR is to fix `svg` element size in Firefox. 
Both with `offsetWidth` and `getBoundingClientRect` FF returns error/wrong result.

I also followed @arian 's suggestion and propose now `.getBoundingClientRect()` as default for `.getSize()`. Was forced to add a exception for IE8- which [cannot calculate](http://jsfiddle.net/R4s8K/) `.getBoundingClientRect()` of elements with `visibility: hidden;`

Added also a spec to cover `svg` element dimensions.

P.S.: When FF bug is fixed and we only support "modern" browsers this `.getSize()` method should be revisited and cleaned up.
You can move this outside of `getSize`. I'd also use `heightComponents` and `widthComponents` instead of an object.

You can also just do 

```js
var gCS = window.getComputedStyle(this), bounds = {x: 0, y: 0};
```