Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit 3daa0171c085cddc9fc2c8dd471de838da6217c8
Author: Tejas Rawal <trawal90@gmail.com>
Date:   Fri Dec 6 15:33:40 2013 -0600

    Add comment

commit 73e45bb1881305ad3143c45b6383f87751b4fdaa
Author: Tejas Rawal <trawal90@gmail.com>
Date:   Fri Dec 6 15:23:52 2013 -0600

    Add trip_distance method