added singleton pattern.

[hoseinfzad]

The Singleton design pattern creates only one object of the class

[gyermolenko]

please review #279

[adihat]

Changes:

def __setattr__(self, name, value):
    return setattr(self.instance, name, value)

Reason: The signature of SingletonObject.__setattr__ doesn't match the signature of base method in class object

[adihat]

@hoseinfzad Please see #256
As @faif mentioned, there is no plan to add singleton pattern to this repo as its considered an anti pattern. There is a plan to maintain a list of these anti patterns in the readme file instead of having to go through all the issues.