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.

Just in case the link to the [example notebook](http://nbviewer.ipython.org/gist/yeesian/1a9d9885a21aa891102d) is down, here's a [pdf](https://dl.dropboxusercontent.com/u/4292151/Washington%20DC%20OSM%20Demo.pdf)
Related to #43.