directions lookup

Add a class/function to look up directions given the coordinates. Then use this class/function in a state to look up directions  for the user.
It would be great to get this into the toolkit to resolve praekelt/vumi-jssandbox-toolkit#192 (it would probably go under [`vumigo.utils`](https://github.com/praekelt/vumi-jssandbox-toolkit/blob/develop/lib/utils.js)). Happy to keep it here for now, but would you be okay with trying out that ticket?