Give a warning when people use $wgArticle. Add's a class that can be used to give...

Give a warning when people use $wgArticle. Add's a class that can be used to give warnings for other globals too.

See discussion on r111168.

Of course no one will see warning because trunk has $wgDeprecationReleaseLimit set to 1.17 by default
(<rant>have I mentioned how I don't like that variable. People enable warnings to be *warned* about
things. Making it so people won't get warned about things until a couple months after we've decided people
should stop using function/interface/etc defeats the purpose of having warnings</rant>)

p.s. Wasn't sure if this waranted something in the release notes, I don't think it does (It's just adding a warning)
but wasn't sure.