BUG: Allow __name__less callables as groupby hows (GH7929)
Allow `functools.partial` objects (and similar callables without `__name__` attributes) to be used as `groupby` functions, which also adds support along the tree (e.g. `resample`, which is specifically tested.)

Closes #7929.

This does not address the potential enhancements allowing automatic naming of duplicate-named functions (relatively minor) and the use of a Series to specify names (more useful).

@dsm054 minor comment, otherwise looks good
 Explicit `return None` and justification for it added; all green.
 thanks!
