DATASOLR-72 - Create schema for entities.

SchemaWriter uses SolrSchemaRequest to read/write schema information from/to the solr JSON schema api. To make this work via SolrJ we’ve introduced according parsers using jackson for json conversion.

We also added configuration options for Java Config and XML which trigger setting the according Features for schema creation. The boolean property is likely to be replaced by a more detailed option letting one configure schema creation features separately.

Automatic core creation is an issue that the current CoreApi does not really allow - at least not without prior interaction on the filesystem.

Moved the external server assumption to a dedicated test rule.

To test against the external server start solr in schema less mode via command line:
  java -Dsolr.solr.home=example-schemaless/solr -jar start.jar