Add a method to calculate trip distance.

[Closes #1]

Squashed commit of the following:

commit 4ea74a249af5935e0620c251ad08986b30392193
Author: Nick Gibson <aki.nicholas.gibson@gmail.com>
Date:   Tue Jul 23 11:35:44 2013 -0500

    completed step 15: added a method to calculate a trip's distance based on duration