changes2html.pl should collect release dates from JIRA REST API

Since LUCENE-3163 removed release dates from CHANGES.txt, the Changes.html generated by changes2html.pl no longer contains release dates, except those older release dates that are hard-coded in the script itself.

JIRA exposes a REST API through which project info is available. For Lucene - java, lots of info is available in JSON format through https://issues.apache.org/jira/rest/api/2.0.alpha1/project/LUCENE , including a full list of each version's release date.