Change demo server

[rCarto]

Change the demo server for https://routing.openstreetmap.de as it allows to request foot/bike/car routes/tables/trips.

This version use hacks to allow different profiles (Used profiles section in https://routing.openstreetmap.de/about.html)

• It implies to adapt the package code to accommodate both profile selection for demo server (via routed-foot/routed-car/routed-bike URL prefixes) and local server (via driving/walk/bike URL prefixes).
  IMO demo server related changes should be invisible to users and profile selection should still pass through options(osrm.profile="walk/bike/driving"). But maybe we should think about another solution to use osrm functions without loading the package (stplanr use-case I think @Robinlovelace ?).
• Update documentation accordingly (link to https://routing.openstreetmap.de/about.html, explicit recommendation about API usage). In readme and package documentation.

[Robinlovelace]

I can have a go at putting in a PR if that would be useful @rCarto.

[rCarto]

Yes, sure!

[Robinlovelace]

First attempt: see commit above!

[rCarto]

Thank you, I'll have a look at it asap. I'll try to drop a new release next week.