Make Dispatcher.register return the undecorated function

This enables stacking the @register decorator.
Improves compatibility with Python 3's singledispatch

https://github.com/mrocklin/multipledispatch/issues/33
Provide Dispatcher.dispatch() method for compatibility with singledispatch

See Python documentation:
    https://docs.python.org/3/library/functools.html#functools.singledispatch

https://github.com/mrocklin/multipledispatch/issues/33