new questions, user_list and user_list_with_geometry

Two new questions
* user_list: generates list of users, with node/way/relation/changeset summary stats
* user_list_with_geometry: generates GeoJSON file per user, with all tagged nodes and ways

and a page on the jekyll site for exploring the results (/users/)

Biggest change to flow of osm-history, is the "multi_users_questions", which outputs a directory of files, rather than a single file.