Add a tag extension <jira> to support JQL

In order to keep backwards compatibility I added a tag extension `<jira>` next to the existing parser function `{{#readjiraissue}}`.

The tag extension takes a JQL as PCDATA and renders the retrieved list of JIRA issues as a table. If an issue has the _Resolved_ status, the issue key is rendered with a strike through. 
I took the way **Confluence** wiki renders JIRA issues as an example.

I've also updated the README.md with the information found on the extension homepage on mediawiki.org: http://www.mediawiki.org/wiki/Extension:JIRAConnector. And added the some describtion about the new tag extension.