Add solr.xml configuration file to support upgrading to solr 4.0

If we want to update to Solr 4.0 and keep the current solr directory structure:

data/solr/
          conf/
          data/

we need to add a solr.xml file in the solr directory which states that the default Solr core we use (collection1) is located in the current (solr) directory. If we don`t do this, solr expects by default that the directory hierarchy is:

data/solr/
          collection1/
                      conf/
                      data/

Since we use a single core architecture, it makes sense to get rid of the redundant "collection1" directory.