Adding explicit support for timed events

[tobiasc]

In the javascript API I can call time_event (https://mixpanel.com/help/reference/javascript-full-api-reference#mixpanel.time_event), but the node API does not have this. Could this be added?

As a work-around I'm calculating the duration manually + added the $duration property to the track event. Based on the logic found here: https://github.com/mixpanel/mixpanel-js/blob/master/src/mixpanel-core.js#L1068

[alekplay]

Bumping this since there has been no activity