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.
Just before this line, maybe something like:

```js
// exported means that the identifier is part of a valid ES6 `export` declaration
```

`preserve` is missing a description too, but since that's not part of this pull request, we can wait to document it separately. (It was implemented [here](https://github.com/jshint/jshint/commit/0673b62b9cdc7927639d1feebe94dcab80a41c6e), but I'll need to research that change further before I can suggest a useful comment.)