visitor log improvements

 * use a limit for the number of actions that are loaded for every visit. without a limit, the web server might crash (e.g. when there are 100K+ page views in one visit after a log import)
 * regular pagination in visitor log. (a) performance improvement: use indexes in the select query. (b) bug fix: the old logic was incorrect. in one setup, there are 10K+ visits but the visitor log only showed one full page and one page with one visit.
 * removed reference to trunk from tests read me