Some changes regarding solr4

See CHANGES.txt:

- Recreate solr config on buildout update if the
  directory of the solr-instance was deleted.
  [mghh]

- New option 'additionalFieldConfig'.
  This option allows for additional configuration options
  in &lt;fields&gt;...&lt;/fields&gt; section of schema.xml
  Use this if you have dynamicFields
  [mghh]

- New option 'directoryFactory'.
  Use it to define the solr directoryFactory for a Solr instance.
  [mghh]