Add integration tests running solr in embedded tomcat

As a preparation for an async solr client I wanted to have integration tests available (with a real solr and http communication), `FailoverLoadbalancedHttpSolrServerIntegrationTest` is added as "showcase".

Integration tests are excluded from `mvn test` execution, to run integration-tests as well use `mvn verify`.

The solr war is deployed to an embedded tomcat (mvn dependency), and downloaded the first time to `$PROJECT_DIR/.solr/solr.war`. The solr home dir (test/resources) is the extracted example/solr directory from a solr 4.6.1 zip.
Right now solr/tomcat is started/stopped for each integration test method, this might be improved (e.g. by using a rule).