org.apache.solr.client.solrj.beans.DocumentObjectBinder customization

I suggest adding a way to customize DocumentObjectBinder behavior. It is not always possible to perform mapping between SolrInputDocument and beans using the default implementation. And SolrServer doesn't have a way to change the default binder for a different implementation. My suggestion is very simple: introduce an interface for binder, and add the ability to set the custom binder for the SolrServer. Please find suggested changes in the attached file. Such an addition will make the solr4J library more flexible.