Create a Apache Solr backend

Write a service implementation that connects to a Solr server for indexing/searching. Should be written so as to also allow other modules to use this (for Solr-specific additions, e.g. "more like this" or auto-correction).

- Probably, all of a server's indexes will be stored with dynamic fields in a single Solr index.
- Some configuration of the schema.xml (and solrconfig.xml?) file will be possible via the server UI.