Add constructor for the `DistanceMatrix` class

- `DistanceMatrix.from_addresses()` takes a variable number of
  `Address` objects and creates distance matrix entries for them
- as a base measure, the air distance between two `Address`
  objects is calculated
- in addition, an integration with the Google Maps Directions API is
  implemented that provides a more realistic measure of the distance
  and duration a rider on a bicycle would need to travel between two
  `Address` objects
- add a `Location.lat_lng` convenience property that provides the
  `.latitude` and `.longitude` of an `Address` as a 2-`tuple`