Add zoom level to osrm request

Adding this parameters has a number of benefits:

1. Returned route geometry will be simplified which saves bandwidth
   (important for longer routes)

2. OSRM will snap the marker to the next non-small connected component.
   Basically that means when you drag a marker on zoomlevel <12,
   OSRM will not snap the marker on a small parking lots, but on the road
   next to it.

To implement this I also had to override the fitSelectedRoutes again,
as we now send a rounting request on each zoom event. I used this
chance to refactor it a little bit.