Streamline & document using additional datasets

[sams96]

It's currently possible to use a data set not included in the package for your reverse geocoding, but I seem to have neglected to document the process and it's a bit annoying since you need to encode the data with the datagen script.

My proposal would be to:

• Remove the datagen script and just store the data in raw geojson
• Remove all but one of the included data sets, to compensate for the increased file size when not using the compression
• Make it really obvious how to use data sets that aren't included, as this would be the way you're expected to use the package.