Multiuser loadtest

Four small features to make this more useful to me:
* Select the login from amongst N users (configurable via querystring arg "nusers")
* Create users, if necessary (based on N)
* Ability to logout (important for UserLog testing), based on configurable probability (via querystring arg "pct_logout" (0...1))
* Configure how likely to do an exercise or video (via querystring arg "pct_videos" (0...1))