Improve error handling

As reported in https://github.com/JOSM/geojson/issues/29 , tag values were imported as represented in the GeoJSON file (e.g. strings were surrounded with quotes).
One consequence is, that now objects and arrays as tag values are discarded, previously their JSON representation was used as tag value.
Also Features that have no "properties" attribute at all are now parsed correctly.

The last node in a way was incorrectly duplicated, now the first node should be reused correctly when a way is closed.

Even if the first and last node are not identical, the resulting OSM ways will loop back to the start node when importing from GeoJSON.

This should solve most of #12 (https://github.com/JOSM/geojson/issues/12). Polygons and Multipolygons are now always imported as closed ways, LineStrings only if first and last node have the same coordinate.

See https://josm.openstreetmap.de/ticket/16850 for the changes in JOSM core that make this possible.
The minimum required JOSM version is now bumped to 14347 because of this change in core.

Mainly for compatibility with JDK 11 and 12, rewrote the build script to the Kotlin DSL.

Add GeoJson download task

The Gradle scripts `version-functions.gradle` (now integrated in `gradle-josm-plugin`) and `dependencies.gradle` (no longer needed, since the Jackson dependency is now gone) are removed.
The `lib/` directory is also no longer needed, since we use jsonp, which is provided by JOSM.

Fix getTags() when properties is null

Add support for compressed geojson

Make jsonp dependencies compileOnly

Remove dependency on jackson and jackson geojson