Crosswalks conversion utility update

The conversion utility now also allow using a geojson file as input to create one of the possible output formats. This simplifies the process of generating a maproulette tasks file in case the initial json file needs to be manually adjusted using some sort of geoprocessing tool.

Using versioneer to provide version

In order to simplify version management, I changed to using versioneer as it uses the latest git tag and a commit delta to get the right version.