Add another Scraper based on an HTML parser

So, this is quite a big change, therefore a branch first.

This refactors the Scraper base class and then adds a new Scraper based on the LXML html parser. I choose that parser because it's fast and very HTML compliant. This allows comic modules to be written using a real parser, currently using XPath expressions to specify "interesting" content.

I used some tricks to make the change as unobtrusive as possible:
- LXML is loaded on demand, so only comic modules using this feature need it (This makes the code a bit more ugly in some places...)
- Helper modules call back into the calling scraper, so they inherit the same parsing logic as the main comic class uses
- When an XPath search results in an HTML element, but we are looking for an URL, we look for a suitable attribute to take the URL from (the list of attributes is sourced directly from LXML, so we don't have to maintain it).

I still have some TODOs open, but I wanted to put this out there, so people can play with it:
- Add cssselect, so module writers can use CSS, which might be more familiar to most people...
- Update documentation to steer new users to use this class instead of the regular expression "parser"

For now, if one needs a quite good XPath reference: http://scraping.pro/res/xpath-cheat/xpath_css_dom_ref.pdf

I resisted the urge to rename the _BasicScraper class, since that would mean changing all plug-in files, making merging harder...