[MRG] Add LxmlLinkExtractor class similar to SgmlLinkExtractor (#528)

~~I had to change the Selector HTML parse from `lxml.etree.HTMLParser` to `lxml.html.HTMLParser` to have those helpful `.make_links_absolute()` and `.iterlinks()`~~

~~There's still margin for some factorisation and cleanup (as much of the code is bluntly copied from `SgmlLinkExtractor`~~

implementation of #528