DOC: add DataFrame et al class docstring to api docs

See #4790

I copied the approach of numpy: extending the autosummary class template to automatically generate all method stub files:
- https://github.com/numpy/numpy/blob/master/doc/source/_templates/autosummary/class.rst
- example output: http://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.html

I added `DataFrame`, `Series`, `Panel`, `Index` and `DatetimeIndex`. Others? (eg `Timestamp`) Or too much?

The only issue at the moment is that normally also a list of attributes should be included, which is not the case (only a list of methods)