Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit c7c663fcb7a7da1749cc6c4c6395073611a582ee
Author: Amy Ton <amyhton@gmail.com>
Date:   Fri Sep 20 10:55:39 2013 -0500

    Add the comment to trip_distance method

commit 5576323c3f12fec00fe0d7809b0cc93b3afc9863
Author: Amy Ton <amyhton@gmail.com>
Date:   Fri Sep 20 10:50:08 2013 -0500

    Add trip_distance method