Use Node.*_NODE instead of defining constants

I think this should be safe, cross browser at this point, but we should make sure. We shouldn't ever be running this code path server side, so I don't think we have to worry about that

``` js
// this
aNode.nodeType === Node.ELEMENT_NODE;

// instead of
var ELEMENT_NODE_TYPE = 1;
aNode.nodeType === ELEMENT_NODE_TYPE;
```
