[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


I will add it to my list of stuff to document: cognifloyd#3