add_helper function to PathElement

[wojtek0806]

This is required to have the following logic also built in for command_mixins:

        key_set = set(kwargs.keys())
        required_minus_received =\
            self._meta_data['required_creation_parameters'] - key_set
        if required_minus_received != set():
            error_message = 'Missing required params: %r'\
                % required_minus_received
            raise MissingRequiredCreationParameter(error_message)

[wojtek0806]

This will also be used here:

https://github.com/F5Networks/f5-common-python/blob/development/f5/bigip/resource.py#L192

[wojtek0806]

added to 0.1.8 closing