Support retrieving z values

[rowanwins]

Hi @mbostock

Following on from #34 I was wondering if you'd be interested in receiving a pull request that provided full support for returning z values from a shapefile?

The GeoJSON spec supports having a z value and if the data is there in the shapefile it would be nice to pass it through to the geojson.

If you're happy to receive the pull request I'm happy to do the work on it :)

Cheers
Rowan

[rowanwins]

Actually I note there is an open pull request at #53 that adds this functionality (noting that I think there are a few things that need to be tidied up in the PR)