Add a 10 minute margin to previous departures

This change is to be somewhat resilient against late departures etc, even though not ideal (as departures *can* be more than 10 minutes late). I often find myself at the bus stop at the exact time the bus is supposed to go, and have to load the previous departures as well. Having this small margin would in most case be sufficient for me.

And thanks a lot for publishing the application, great learning experience for me in meego development :-).