Don't warn on unused exports + Do warn on export declarations outside global scope

It's really two separate bugs, but just one patch! It looks to me like export
declarations outside of "global scope" (or whatever the module scope is), is a
syntax error --- traceur will report it as such, so we should probably throw up on it.
it's legal in traceur, but that doesn't necessarily mean much. since there are no real module implementations yet, I just don't know