Add autoTrack session duration

[ohlr]

Is your feature request related to a problem? Please describe. https://stackoverflow.com/questions/42355311/pinpoint-session-duration

Pinpoint does not support session duration.

Describe the solution you'd like I wonder if it is possible to compute the session duration client side using the autotrack functionality

The difficult part would be I think to store the session start in local storage.

Edit: Actually the duration is already computed client side. Why is this not reported in Pinpoint then?