Add markers (start and end) to trip plans on map

[barbeau]

Summary:

Currently trip plans are just lines on the map:

IMHO, this makes it difficult to tell which end is the origin or destination, and also makes it difficult to tell where there is a transition between two routes, or between transit and walking.

I'd like to add markers to make the origin/destination easier to distinguish, and markers to indicate mode transitions.

Here's how the markers look in OpenTripPlanner Android:

Here's how the markers look in Google Maps (left is using real-time location as origin, right is using a static location for origin):

I like the Google Maps approach for the origin/destination, with the origin as a circle (gray if static, blue if real-time), and the classic red marker for destination.

For mode transitions, something like OTP Android may work. We should use material design icons, though.

[barbeau]

I've added simple start/end markers in 0cac62a, so it now looks like:

I opened #755 to deal with mode transitions, etc.