Additional fixes for Solr >= 4.0

Added "softCommit"
(http://wiki.apache.org/solr/UpdateXmlMessages#A.22commit.22_and_.22optimize.22)
Replaced "allowDups", "overwritePending" and "overwriteCommitted" with
"overwrite"
(http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22add.22)
Updated phpdocs
Merge pull request #1 from ilalmi/master

Additional fixes for Solr >= 4.0