fixes issues with tagName checking on svg elements

I noticed that when creating a svg via `document.createElement('svg')` the tag name is set to 'SVG' rather then 'svg' and when it goes through the wrap function, inside `lib/svg.js` the check is `dom.tagName == 'svg'`. This made Snap return a `Element` without a `Paper` object.

The environment this occurred on was Chrome 31.0.1650.63, Ubuntu