Add per-waypoint timestamp option for map matching

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The Map Maching API has an optional [`timestamps`](https://www.mapbox.com/api-documentation/#retrieve-a-match) query parameter. Currently, this library doesn’t offer a way to specify that parameter. Waypoint should have a `timestamp` property (of type Date). `MatchOptions.getter:params` should aggregate the `timestamp`s of the waypoints into the `timestamps` parameter.

In https://github.com/mapbox/MapboxDirections.swift/pull/236#discussion_r172037100, I had suggested relying on `CLLocation.timestamp`, but it turns out that Waypoint stores coordinates and headings separately, not as part of a location. That should change, though not necessarily as part of fixing this omission.

It isn’t clear to me what should happen when one waypoint has a timestamp and another does not. Maybe this semicolon-delimited list can contain blank entries.

/ref https://github.com/mapbox/mapbox-navigation-android/issues/875#issuecomment-421332368
/cc @frederoni

Start by reading the Waypoint type and MatchOptions.getter:params, then trace how map-matching query parameters are assembled. Add the timestamp option and define how missing waypoint timestamps are represented; done means the resulting timestamps parameter is correctly generated and covered by the relevant request tests.

Written by the indexing model from the issue text.