:meth:`.WMessengerOnionProto.layer` method implementation.

def layer(self, layer_name):
		""" :meth:`.WMessengerOnionProto.layer` method implementation.
		"""
		if layer_name in self.__layers.keys():
			return self.__layers[layer_name]
		elif layer_name in self.__class__.__builtin_layers__:
			return self.__class__.__builtin_layers__[layer_name]

		raise RuntimeError('Invalid layer name')