Keep ppl.subplot{s,2grid}.__module__ correct.

functools.wraps optimistically sets the __module__ attribute of these
functions to mpl.pyplot, but we only want to set __doc__ and
__wrapped__, so that the functions appear in pydoc.

Fixes #65.
Merge pull request #66 from anntzer/show-wrappers-in-pydoc

Keep ppl.subplot{s,2grid}.__module__ correct.