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.
okay --- i've added a new warning if a `let` is exported before it is defined --- and there's a note in the test that it should do the same thing for `const`