Add back traceurRequire

This adds back a way for Traceur to provide a require for Node.js

This now compiles the required file using the CommonJsModuleTransformer, which means that recursive imports will only work if makeDefault has been used.

This feature was removed in #604

Fixes #722

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/723

Closes #723, closes #722.