Added a new method to calculate distance.

[Closes #2]

Squashed commit of the following:

commit 5afeff0e1b5fbc7a327898ed35756a22b1de602d
Author: Matt Buck <github@matthewbuck.com>
Date:   Tue Jul 23 12:09:39 2013 -0500

    Added method to calculate distance.

commit 0dc5093f0e655ac0f7b19341f7b5d8bd82fa7613
Merge: 78a8354 b07aaf4
Author: Matt Buck <github@matthewbuck.com>
Date:   Tue Jul 23 11:20:46 2013 -0500

    Merge branch 'master' into calculate-trip-time

    Conflicts:
    	train.rb

commit 78a83540cedb5ae054be097149e9b28991e2f824
Author: Matt Buck <github@matthewbuck.com>
Date:   Tue Jul 23 11:15:00 2013 -0500

    Add CORRECT method for calculating duration.