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 adds the following shim to the parseGexf plugin:

```javascript
var getElementsByTagNameNS = function(node, ns, tagName) {
	if (node.getElementsByTagNameNS) {
		return node.getElementsByTagNameNS(ns, tagName);
	}

	return node.getElementsByTagName(ns + ':' + tagName);
};
```

It's then called like this, for example:

```javascript
positionNodes = positionNodes.length ?
                      positionNodes :
                      getElementsByTagNameNS(nodeNode, 'viz','position');
```

Tested in IE9, Chrome, Firefox and Safari.