`add_multiple_results.py` fails

Using 47d29e5a installed under Python 3.5 I find that the add_multiple_results.py example fails. It seems that the __add__ method of the Results() class is failing as it can't find a samples attribute in the class. Digging a bit deeper, somewhere between samples attribute being set here and the samplers being added the samples attribute is being deleted by samples_to_posterior().

Shoud the __add__ method really be more like:

def __add__(self, other):
    matches = ['sampler', 'search_parameter_keys']
    for match in matches:
        # The 1 and 0 here ensure that if either doesn't have a match for
        # some reason, a error will be thrown.
        if getattr(other, match, 1) != getattr(self, match, 0):
            raise ValueError(
                "Unable to add results generated with different {}".format(match))

    if not hasattr(self, 'samples') and not hasattr(other, 'samples'):
        if not hasattr(self, 'posterior') and not hasattr(other, 'posterior'):
            raise ValueError("Unable to add results")
        else:
            self.posterior = pd.concat([self.posterior, other.posterior])
    else:
        self.samples = np.concatenate([self.samples, other.samples])

    return self