Promote LxmlLinkExtractor as LxmlExtractor

on top of the amazing work of @redapple in #559, this PR includes an alias to LxmlLinkExtractor importable from `scrapy.contrib.linkextractors.LinkExtractor` and update docs. 
It seems this directive will be overwritten by the next directive, so the docs would be for `scrapy.contrib.linkextractors.sgml.LxmlLinkExtractor`.