Insert wrapGenerator.mark call after "use strict"

Fixes #88.

Test Plan: Run

    node -e "console.log(require('./main.js')('\\'use strict\\';function *range() {}'))"

and observe that the mark() call is after the "use strict" line.
Merge pull request #105 from spicyj/gh-88

Insert wrapGenerator.mark call after "use strict"