functionality for finding nodes within range

To (eventually) provide [drivetime isochrones](http://anitagraser.com/2011/02/12/drive-time-isochrones/).

Here's an [example notebook](http://nbviewer.ipython.org/gist/yeesian/1a9d9885a21aa891102d) to demonstrate how `nodesWithinRange`, `nodesWithinDrivingDistance`, and `nodesWithinDrivingTime` can be used.

Related to #43.