TNG-41: upgrade ambra tests to solr 4.1.  This was complicated by the fact that the solr tests themselves were refactored between 4.0 and 4.1.  Some of our tests use their test infrastructure, so I had to rejigger things.
TNG-41: add support for stopword files.  This allows us to use the production solr schema in our ambra tests.
TNG-41: use the same solr config file as we do in production, with the exception that we don't specify a data directory since this is handled differently in EmbeddedSolrServerFactory.
Merge branch 'solr_tests' into jan_r2
Merge branch 'jan_r2' of https://github.com/PLOS/ambra into jan_r2