Level 2: Add support fot NodeFilter constants

Add [DOM Level 2 Traversal: NodeFilter](http://www.w3.org/TR/DOM-Level-2-Traversal-Range/traversal.html#Traversal-NodeFilter).

For some reason no assertions of the w3c test suite are detected by the test runner, so I added them as well to the level2/core tests. I also fixed some JSHint issues.
Yeah exactly. Probably best to create a utility function, perhaps in the utils file, so you can do

```js
addConstant(NodeFilter, "FILTER_ACCEPT", 1);
```

and it'll do the defineProperty fun times for you on both `NodeFilter` and `NodeFilter.prototype`.

Heck, maybe even

```js
addConstants(NodeFilter, {
  FILTER_ACCEPT: 1,
  ...
});
```