Wrapper do not copy docstring

[rqelibari]

mitogen.utils.with_router does not copy the wrapped functions docstring. Many libraries (e.g. click) rely on it though.

from mitogen.utils import with_router

@with_router
def some_click_command(router):
    """This is the description of the command."""
    pass

assert(some_click_command.__doc__ == "This is the description of the command.") # raises exception