Allow events of limited_public groups to be listed

Groups which are marked "public_limited" do not appear in the API query results by default, however they still display useful public information about upcoming meetups. (e.g. London http://www.meetup.com/PyLadiesLondon/)

This pull request adds the limited_events=true parameter to the API query so those meetups can be included. No event URL is returned by the API in this case, so there's also a change here to link to the group URL.