[WIP] Custom namespaces

An extended implementation of https://github.com/Masterminds/html5-php/pull/44 that supports also custom namespace prefixes and XML style namespaces.

See here to see an example: https://github.com/goetas/html5-php/blob/custom-namespaces/test/HTML5/Parser/DOMTreeBuilderTest.php#L99


Could you include a comment about why you're doing this (a comment on nsStack would be ok)? It was not immediately obvious to me how nsStack worked.

After study, I think that: ``$this->nsStack[0]['']`` is always one of the nsRoots. ``$this->nsStack[0]['prefix']`` is any of the implicit namespaces defined in the html spec or defined when creating the DomTreeBuilder. These are the prefixes that are just available. right?