Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit 896868f8cd7d2471499d420c15bd2e5af293c79e
Author: Jesse Kevon <jessekevon@Jesses-MacBook-Pro.local>
Date:   Fri Sep 20 15:07:33 2013 -0500

    add trip distance method