Add extended PackageInfo.g validation

[fingolfin]

The online validator is gone. To replace it, let's add code to ReleaseTools to do so. I have this in mind:

• a new validate script. If run w/o args, it'll look for a PackageInfo.g in the current dir; read it; download the PackageInfo.g mention in the URL; then report the version/date in that; and proceed to test it (check all URLs in it, download archives, extract them, compare their PackageInfo.g files to check they are identical to the one downloaded; etc. -- everything the online validator did)

• in an alternative mode, this script would not download the remote PackageInfo.g, but still perform all the checks, albeit only with the local PackageInfo.g. This can be used after release has been run, but before the gh-pages branch has been pushed, i.e. to verify everything is fine before updating the online PackageInfo.g

Of course this script would be useable even for people who don't use ReleaseTools.