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.
This test seems redundant with the web-platform-tests, so no need to add it :)