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.
My eyes scanned right by this, but yes—this would be illegal, as those bindings would attempt to resolve before the declaration is reached.