Resolve release date from tag on GitHub

[gkossakowski]

If you leave out the release date parameter for MakeDownloadPage it'll just take current date. That's probably not what you want because we should use date of when the tag was created.

It's possible to resolve it using GitHub's API:

curl -s https://api.github.com/repos/scala/scala/git/refs/tags/v2.11.0-M5 | json object.url | xargs curl -s | json tagger.date
2013-09-07T00:05:56Z

assuming the jsontool is installed.

//cc @retronym