DS-2297: Auto upgrade Solr indexes

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)

We should ensure that our differences in Lucene vs Solr versions doesn't cause a problem with this script, as this script is using the *LUCENE* API to determine an index directory version...so it may or may not work well if Solr automatically upgrades that directory to a "future" version of Lucene.
I really don't see this method as all that verbose overall. I'd rather not muck around too much with something that "works" in order to try and achieve slightly less verbosity.  Plus, I have to admit, I'm working under some tight time constraints here. Someone else is welcome to rework this if we really see it as high priority (and the unit tests will tell you if you are doing it wrong). Otherwise, I'd say let's leave this as-is for now, as it works perfectly fine.