Correct make dependencies for src/codegeneration/modules and to fire npm install on make clean.

Sync up bin/traceur.js

Review URL: https://github.com/google/traceur-compiler/pull/687

Closes #687.
Simplify generated generator code

- Instead of storing things in the closure we store it on a context object.
- Remove the $G object
- Use a sentinel for termination
- Extract async function code into a runtime function to reduce code duplication.