Add `initWithLeg(Leg leg)` support to `BusStage`

[iswheeler]

The BusStage should accept a new initWithLeg(Leg leg) function that takes in a Leg object (that contains data about the stage--such as bus stops, start/end times, etc.) and import it into your stage so it's ready to go with the data from the Leg.

[jumpy-cat]

Should initWithLeg be a regular method or a static/factory function? (I assume regular method?)

edit: I see your example now, it will be a regular method.