DS-2297: Auto upgrade Solr indexes

https://jira.duraspace.org/browse/DS-2297

This PR adds the following:

* A new Ant target (update_solr_indexes) which automatically upgrades Solr indexes as specified using a few other new targets
* A new "IndexVersion" java class (and unit tests) which can determine the Lucene version used to create a specific index directory, and also determine if that version of Lucene is older/newer than a specified version.

This needs further testing, but with these in place, simply running "ant update" will automatically check your Solr Statistics and OAI-PMH indexes (if they exist) and first upgrade them to Lucene 3.5.0 (if needed) followed by an upgrade to Lucene 4.9.1 (which is what we are using). This follows the strategy at: https://wiki.duraspace.org/display/DSPACE/DSpace+Release+5.0+Status#DSpaceRelease5.0Status-Solrupgrade(WIP)