Move services and libs out of app/models/

Simple reorg to keep AR classes under app/models. Easier to grasp what the domain objects are this way, IMO.

ScrapeDriver went into lib/, since it's an utility class. WeatherMan, ReadingScraper and PDFWriter into app/services, even though the first two are only used in a rake task. Felt like these are important enough actors to be visible within app/. This is subjective, of course - there's an interesting discussion, if somewhat old, about where to stash code [on Code Climate's blog](http://blog.codeclimate.com/blog/2012/02/07/what-code-goes-in-the-lib-directory/)

Also added some coverage to users_controller#download_pdf.

This PR includes changes from #8 and #9, so better to merge (and review) it after them.