SolrEntityProcessor improvements

I'm working on a couple of SolrEntityProcessor improvements:

1) ability to provide a specific HttpClient instance to make authentication, https connections and so on.
2) introduce consistency between solr parameters and parameters used from SolrEntityProcessor to query Solr: better rename fields to fl and format to wt while we can (the processor has not been released yet). Better use CommonParams constants instead of new ones with same content.