Checking release date

[olexandr-konovalov]

Suggestion by @lhsoicher: why not to check release date in PackageInfo.g while running ReleaseTools?

Leonard suggests that one should be able to override it (I can envisage situations when that happens, e.g. when one publishes a release for someone else), but by default it could stop with an error message telling that the release date in PackageInfo.g is different from today's date.

[fingolfin]

Duplicate of issue #21. Patches welcome :-)