updates Ractive to the latest version, unminified
adds prefix parameter to Router to allow Routers that only deal with subdirectories

e.g. new Router("/play")

this will only ever match routes that start with /games, and the code using
it deals with whatever comes after it - if the url is /play/game/123 then
/game/:id will be matched and getCurrentPath will return /game/123.  for
/play/game, getCurrentPath will return "/", not "".  for /blah/somethingelse,
it will return null.