surface, tracktype, smoothness tags in car profile

thanks to @ftrebien for providing an initial PR at https://github.com/Project-OSRM/osrm-backend/pull/955. 

the current PR uses a simpler way to handle multiple tags: it simply picks the slowest speed mandated by the tags present as well as the road type.

actual speeds have been estimated from the photos at the relevant osm wiki pages, but could be discussed.
Then it should be set to a value like +infinity. 400 is somewhat arbitrary and hides the intent of the value.