Support individual field updates introduced with Solr 4.0

update = "add" | "set" | "inc"
Explanation: http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22field.22

Both current and future field update types supported in a generic way.

Example usage:
solrConn.add(docs, fieldUpdates={'myfield1_ss': 'add', 'myfield2_s': 'set', 'myfield3_i': 'inc'})