Preserve generators (facebook/jstransform#20)

This change preserves generators in the class and concise method transform.

In the class transform, the `*` was just ignored, meaning that the function definition was converted to a "normal" function.
The concise methods transform actually produced invalid syntax, since it kept the `*` in the wrong place.

The result can be converted with facebook/regenerator#master to make it truly ES5/ES3 compatible.
I'm wondering whether these tests are "good" enough. I originally loaded regenerator to convert the result and evaluated it. But it felt a bit "heavy".