Add the ability to specify the release date when adding a new Extension version

It would be nice to be able to see when a release was done by looking at the Release Notes section of an extension. For example looking at http://extensions.xwiki.org/xwiki/bin/view/Extension/AntiSpam+Tool+Application#HReleaseNotes I have no idea when version 1.5 was released.

As a user, this is an information that I look at when I check release notes of software I want to use.