Adds object to list of grindstone['tasks'].

def add_task(self, name=None, desc=None):
        """
        Adds object to list of grindstone['tasks'].
        """
        # A name is required to create a task
        for k in self.get_tasks():
            if name in keys_of(k):
                raise ValueError('Task already exists')

        if name is not None:
            # desc can be None, so we can just append whatever we have
            self.grindstone['tasks'].append( {name: desc} )
        else:
            # Raising errors is good, and makes tests easy.
            raise ValueError('Tasks `name` cannot be None')