Update navigational metaphor

[elliottwilliams]

Quoting the 1.0 MVP:

Routes go on the root navigation stack. Stations are modals that appear above the navigation stack. Stations can be "minimized", and multiple station modals can be opened at once (see functionality in Mail or Fantastical for examples). When a route is selected from a station view, the station modal is minimized, and the route is pushed after the minimization animation completes.

• Write or include a library for minimizable modal views.
  • Ensure that there's no native way to do this in iOS, i.e. if Mail uses a public API.
  • Add a minimize button to minimizable modals.
• Modify the station view controller and its header bar to be such a modal.
• When station view controller is minimized, show its name and upcoming route badges, similar to the design in Show upcoming arrivals on stations #11.