[WIP] Drivetime Catchment Areas

This is mostly to get a repo-level diff, and to collect all remaining work. In particular:

- [x] update tests
- [x] documentation for `nodesWithin*` (see #51 )
- [ ] plotting for `nodesWithin*`
- [ ] interpolation methods for drivetime regions (optional)

~~I'm working on the documentation now, but am unsure of the appropriate terms; we can bikeshed after I get something up? In particular, I'm thinking of "drivetime regions", but [catchment areas](http://anitagraser.com/2011/02/09/creating-catchment-areas-with-pgrouting-and-qgis/) (or some other name that's more agnostic to the mode of transportation) could work too.~~

It'll be great if you can provide the plotting functionality for the drivetime regions. I'm not sure how best to represent them, so anything else you have in mind is probably better than what I had in [my example](http://nbviewer.ipython.org/gist/yeesian/1a9d9885a21aa891102d). And yeah, it'll be great to have it in the album; if you wish, I can add one for Washington DC, once we have the plotting functionality up.

Moving forward, I think what's left is the provisioning of [interpolation methods](http://docs.qgis.org/2.2/en/docs/gentle_gis_introduction/spatial_analysis_interpolation.html), to process the nodes into regions/polygons. It isn't a OSM-specific feature (neither were the routing/drivetime/plotting/simulation functionality), but I can open an issue to look into the generation/plotting of [contours](https://github.com/tlycken/Contour.jl) or polygons (through "alpha-shapes"/concave-hulls), if it's of interest.