Support individual field updates as introduced with Solr 4.0

Currently supporting:
update = "add" | "set"
as explained in: http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22field.22

Example usage:
solrConn.add(docs, addUpdates=<list_of_fields_to_add>, setUpdates=<list_of_fields_to_set>)