Mixpanel: track page views as a single event, and allow optionally disabling of page view tracking

I just started using Mixpanel and found it was logging all kinds of events because pageviews were tracked as distinct event types.  This commit fixes that, tracking all pageviews as a single event type.

I'm a little hesitant to request you pull it in, as it will break compatibility for people who rely on the current behavior.  However I can't really imagine anyone doing Mixpanel pageview tracking via analytical.track without this sort of a change.

If you'd like I can change the options around to preserve the old behavior as the default.