Added __reduce__ method to IrregularlySampledSignal

IrregularlySampledSignals were not being pickled properly because the signature of the __new__ method had changed from the inherited Quantity class and so the supplied pickling function did not work (see Issue #155, https://github.com/NeuralEnsemble/python-neo/issues/155). Copying the approach taking for the AnalogSignal class I have implemented a __reduce__ method for the IrregularlySampledSignal class.