Added Presence start-end view.

https://gist.github.com/przemekmazurek/581b5c4c9fa4931bfa5f
I'd split this return in 2 parts---it's not as readable as it could be. First, compute the mean, then return it and deal with empty  `items`.

You can test for `items` at the beginning (early exit) or at the end. Choose one or the other and tell me why later.