Better, more future proof Fiona and Shapely usage

Hi, Sean Gillies here, the author of Fiona and Shapely. I'm very pleased to see them being used in your project! Shapely is a spin off of the [Pleiades Project](http://pleiades.stoa.org), a gazetteer of the ancient world. Gazetteers helping gazetteers :)

This pull request will future proof your ETL scripts. The `shapely.geometry.asShape()` and `fiona.collection()` methods aren't for developers. I've replaced them with calls to `shapely.geometry.shape()` and `fiona.open()`, documented methods of the public APIs of those packages.