[WIP] Add support for class member variable

I've submitted this PR as WIP to get feedback, **the code does not work for now**

I am looking for:
- feedback on the implemenation,
- how to debug (currently I'm doing make + node debug which is not the most user friendly workflow),
- what kind of test should be added.

/cc @arv
This should be `super.transformClassDeclaration(tree)`. The spec changed and I'm in the process to change traceur.