Resolve strict mode and module.exports

Both `“use strict”;` and `module.exports = function () { … }` introduce 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.