Add a method to calculate trip distance.

[Closes #2]

Squashed commit of the following:

commit 6aca4500bb04fba426df2f9c1997981c9be4fb0c
Author: taylor smith <twsmith89@gmail.com>
Date:   Fri Sep 20 11:11:11 2013 -0500

    add commenting

commit 782cc9ffd7bc9beb9685084e83ece74ab55a8a81
Author: taylor smith <twsmith89@gmail.com>
Date:   Fri Sep 20 11:08:50 2013 -0500

    add train#trip_distance