Support wrapGenerator.isGeneratorFunction

This is my current best effort to resolve https://github.com/facebook/regenerator/issues/48.

These changes should also support implementations of `isGeneratorFunction` that don't have access to the `GeneratorFunction` constructor for direct comparison, such as https://github.com/visionmedia/co/blob/c247a120e3/index.js#L201-L211.

cc @TooTallNate @Raynor