Resolves #813

This feature is actually a part of the Level 4 specs (https://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#parent-element). I put the implementation and the tests into the level 2 folders though, because I didn't want to create a whole new structure for just one attribute, especially if you guys plan to get rid of the level spec structure anyway (according to the guidelines).
I think it would be best to just check `.nodeType`. Also I think `core === core.dom.level3.core`?