Associate stops with routes

[Gerzer]

Currently, stops are represented in an entirely disconnected manner from routes. We should somehow associate each stop with one or more routes. Since the physical location of a stop is usually not directly on top of any route’s poly-line, we’ll need to give each stop a distance-along-route measurement (similarly to what will need to be done for buses in #54).

How to perform this association is an open question. We could dynamically compute it at runtime, or we could specify some way of hard-coding it in the GPX files. However this is done, the information should be communicated back to clients in response to relevant HTTP requests.