Added JSON update support for Solr versions >= 3.1.

[nhooey]

Improves document serialized speed by about 3x.

[nhooey]

The comment added here:

•    # Solr's JSON update handler takes multiple "add" commands for
  

•    # multiple docs, unlike the XML version, so we have to serialize
  

•    # an "add" JSON object for each document, then remove the leading
  

•    # and trailing curly braces
  

Applies to code designed to work with Solr <= 3.1. Solr >= 3.2 doesn't need this workaround, but works with it. This bug was fixed in https://issues.apache.org/jira/browse/SOLR-2496 .