Removed an unused signature of verify_roles().
Simplified module exports.
Improved initial config.

I figured that roles must be more of an erlang or javascript array of strings
because it looks more like the other config there is, and it isn't hidden in
the futon config page (in contrast to undefined config vars).
Adapted string tokenization to new roles.

This is more of a hack until I figure out how to parse these arrays. I'll
probably just use ejson.
Added more useful ogr2ogr configuration.

They are as explained in the comment. I have no useful guess for GDAL_DATA, so
again, I enforce it to be set before the plugin is first used.
Changed validate_config to check all undefined parameters.
Replaced get_command with get_config("command").
Implemented use of GDAL_DATA and fallback_crs.

In a first step, remove-me.geojson is written by ogr2ogr and the plugin checks
if it also contains a 'crs' property, in which case the GeoJSON can be returned
to the client.
If the property is missing, we provide GDAL_DATA in env and transform all
coordinates to the given fallback_crs, to ensure that we end up with the crs
property.
Removed some diagnostic log.
Simplified get_allowed_roles().

Because validate_config requires it to be defined, and I provide a meaningful
initialization anyway.