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. 
I'm not fond of starting LxmlLinkExtractor docs with a reference to (supposely inferior, and maybe pending deprecation) SgmlLinkExtractor. Maybe something like this?

> LxmlLinkExtractor is the recommended link extractor with handy filtering options. It is implemented using lxml's robust ``HTMLParser``.