Remove all references to Attr.ownerElement...

According to MDN:

This property has been removed from Firefox 29. Since you can only get
Attr objects from elements, you should already know the owner.

-- https://developer.mozilla.org/en-US/docs/Web/API/Attr
Merge the "Output" section of README.md into the "DOM API" section
Exclude test/git-hooks directory and CONTRIBUTING.md from git archives
Merge branch 'master' of https://github.com/craigbarnes/lua-gumbo
move rockspec check to separate rule