Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit b561e43a9a1c9fd3bacecc81103faea5fa5ea1e0
Author: John Caldwell <caldwelljs@gmail.com>
Date:   Fri Dec 6 15:43:53 2013 -0600

    Add comment to trip_duration method in train.rb

commit e55cb3c1d52ea0bb2e2f22919d393c7850e71e5a
Author: John Caldwell <caldwelljs@gmail.com>
Date:   Fri Dec 6 15:37:31 2013 -0600

    add trip_distance method to train.rb