Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit 579a7f5f33496ef7cf3ba8b69cd4a97a3666e11a
Author: Alex Levine <alexlevine@lab-2.att.net>
Date:   Tue Jul 23 11:33:29 2013 -0500

    Added a distance function