Trait introspection

A class with multiple traits usually has an extremely useless call signature of `__init__`, and is often lacking docstrings. This is understandable, since traits themselves can come with descriptions.

On the other hand, it would be useful to automatically document the traits present in the class in the docstring by e.g. using a decorator, or even fiddle with the `__init__` call signature, since that would enable tools like `sphinx` to produce more complete documentation, and would allow introspection by third party software (unlike the related ipython/ipython#9000).