Add a _proxyImpl helper class

If you can come up with a better test for this, that'd be even nicer.
(Formalchemy sure does some other somewhat-ugly things under the hood …)

formalchemy does the following:

        if isinstance(self._impl, ScalarAttributeImpl) or self._impl.__class__.__name__ == '_ProxyImpl': # ProxyImpl is a one-off class for each synonym, can't import it
            # normal property, mapped to a single column from the main table
            prop = getattr(self._property, '_proxied_property', None)
            if prop is None:
                prop = self._property
            return tuple(prop.local_columns)

With this patch, this test can be replaced with

		if isinstance(self._impl, (ScalarAttributeImpl,ProxyImpl):
            …