Add description of `config.headers`
Removed a lot of unused requires
Refactor DOMWindow into its own file
Rename DOMWindow to Window
Moved lots of properties to level1

This is to reduce monkey patching these properties afterwards.

If we need an object, we should define it in a single file, not
distribute its code over 5.
Move nodeName to Element prototype
Fix casing issues in level1 tests
Remove horrible tests from a bygone era
Remove tests which fail due to DTD checks
Remove even more tests testing random attributes
Fix casing issues in level2/core
Remove "default attribute" related tests from level2/core
Remove level1/svg tests!

They were literally the exact same tests as the level1/core tests, except with a different xmlns attribute on the document root.
Move parentWindow setter to level1
Removed all traces of levels

The only thing remaining is the defaultLevel export. I left that in for
now in order to allow someone to overwrite internals without having to
resort to require()ing deep into our internal structure.
Only apply the selector functions once
Remove defaultLevel from public API too
Move html parsing into level1 too

Sadly, this creates even more private variables on the dom which we have
to get rid of.
Add a readme explaining what's going on in lib/
Remove all Object.create usage on the levels

Now all the level code is modifying the same object, which is good.