Support viz:* in GEXF in IE

As mentioned in #16, #59, #45 and #73 IE doesn't support `getElementsByTagNameNS`, which makes it impossible to get the `viz` namespaced attributes like `viz:color`, `viz:position` and `viz:size` *the right way*.

**However**, it is still possible to retrieve namespaced nodes using `getElementsByTagName` by passing the full node name, including the namespace, as an argument.

This pull request changes the plugin so that all viz namespaced elements are retrieved the following way, which works perfectly in IE9, Chrome, Firefox and Safari.

```javascript
nodeNode.getElementsByTagName('viz:size');
```