Add a method to calculate trip distance.

[Closes #3]
Squashed commit of the following:

commit 2166598dde2171b2f3ae1678cf8ca7de5d0ce99d
Author: MakerSquare Student <students@themakersquare.com>
Date:   Tue Jul 23 11:53:55 2013 -0500

    Added comment to duration method

commit 4bb051743e641a4675200bd36c2078ef2b58d59e
Author: MakerSquare Student <students@themakersquare.com>
Date:   Tue Jul 23 11:42:54 2013 -0500

    added duration method