Post-0.1 directions screen improvements

[ellenhp]

Following up from #121. That issue is specifically scoped to 0.1, so this new issue will track additional improvements post-0.1.

• Allow drag the origin and destination markers. Nicer if the route is recalculated in near real-time while the user is dragging it, like in Gmaps.
• Allow the user to add intermediate points.
• Allow the user to drag the polyline to change the route.