Cleanup initialize and how it calls initialize_layers

[cancan101]

See:

nolearn/nolearn/lasagne/base.py

Lines 285 to 287 in b4a4cb6

	out = getattr(self, '_output_layer', None)
	if out is None:
	out = self._output_layer = self.initialize_layers()

out is not used nor is self. _output_layer other than to see that initialize_layers has been called.

Further it would be nice to move away from the assumption of just one output layer.