Add shortest distance to WMATA api routes result

[NealHumphrey]

The alpha version of the api has a nearby_transit endpoint that returns:

• List of all bus stops and metro stops within 0.5 miles walking distance
• Unique list of bus routes and metro routes accessible at these stops

As an enhanced version, also return the shortest walking distance to each route.

For the 'bus_routes' and 'rail_routes' json results, instead of returning a list of strings, return a list of objects:

"bus_routes": [
    {'route': "30S", dist_in_miles': "0.02"},
    {'route': "D51", dist_in_miles': "0.23"}
  ]

Easiest implementation of this might be:

• Use the 'unique_transit_routes' function inside the for loop to get a list of unique routes at a stop. Add this to the dictionary as additional information returned since we have it.
• Store the shortest distance for each new route. If the distance is shorter than a previously stored distance, update the route to shortest distance mapping in the for loop.

This ends up replacing the two calls to unique_transit_routes at the end, since we're doing it as we go in the for loop.

[NealHumphrey]

Resolved with #211