Integration of the GeoStyler plugin

[dnlkoch]

With #61 the GeoStyler plugin is taken from the official repository at https://github.com/geostyler/geostyler-geoserver-plugin while building the image.

Unfortunately the existing packages can't be used without authentication, thus the plugin is going to be compiled each time. This should be fixed by providing prebuilt artifacts from the GeoStyler (e.g. by attaching release artifacts).

[buehner]

The custom build of the geostyler geoserver plugin significantly enlarged the size of the resulting docker image: https://hub.docker.com/r/terrestris/geoserver/tags

This can probably be fixed by cleaning up the build resources after a succesfull build.