add a method to calculate trip distance
[Closes #2]

Squashed commit of the following:

commit b3a60d8f7a8389cd330c473885421cc3eaec271d
Author: Thomas Seals <tjseals@gmail.com>
Date:   Fri Dec 6 15:37:16 2013 -0600

    fixed trip duration method

commit a6e94bc618248cb4298df171f533e1e24e136528
Author: MakerSquare Student <students@themakersquare.com>
Date:   Fri Dec 6 15:25:26 2013 -0600

    Add trip distance method