Add the Importer extension to the WWSK build

[emxsys]

From the GeoServer Importer extension page:

The Importer extension gives a GeoServer administrator an alternate, more-streamlined method for uploading and configuring new layers.

There are two primary advantages to using the Importer over the standard GeoServer data-loading workflow:

• Supports batch operations (loading and publishing multiple spatial files or database tables in one operation)
• Creates unique styles for each layer, rather than linking to the same (existing) styles.

See: http://docs.geoserver.org/latest/en/user/extensions/importer/index.html

[emxsys]

The jar file for the org.geoserver:gs-importer is not in the boundless maven repository.