Resolve strict mode and module.exports

~~Both `“use strict”;` and~~ `module.exports = function () { … }` introduces a bugs. This commit ~~always removes the first “use strict” (although this doesn’t touch non-Traceur code) and~~ renames `module.exports` to `harmony` if Traceur injects it.

~~To reproduce the "use strict" bug:~~

```js
foo = 'bar'; // ReferenceError: foo is not defined
```

To reproduce the `module.exports` bug:
```js
this.foo = 'bar'; // ReferenceError: module is not defined
```

This pull request resolves #4 and resolves #5.