Implement minDate/maxDate filters

Implementation of https://github.com/maiera/gde-app/issues/39

minDate/maxDate parameters have to be supplied as String in YYYY/MM/DD format (same format as post_date is stored in).
Other values won't cause a problem either because post_date is a StringProperty (not sure if we want to keep it like this or use DateProperty instead?) so string comparison is used. It's also possible to use e.g. maxDate=2013 to get all activities up to the end of 2012. (since '2012/03/31' <= '2013' but '2013/01/01' > '2013')