Handle the "bits" type

[fperrin]

The YANG "bits" type gets mapped to a Set[str] object in Python. The set object is augmented with a dictionary keeping track of the bit positions. This is used when forming the JSON or XML serialisation.

closes #314

[JoseIgnacioTamayo]

Awesome! Thanks.

I did not like the name 'YANGBitsFactory', but I could not think of a better name for a method that creates a class. So it is fine by me.

[fperrin]

That can be renamed to YANGBitsType returning YANGbits to follow the same pattern as other types. I'll do that now

def RestrictedPrecisionDecimalType(*args, **kwargs):
    """
    Function to return a new type that is based on decimal.Decimal with
    an arbitrary restricted precision.
    """
    precision = kwargs.pop("precision", False)

    class RestrictedPrecisionDecimal(Decimal):
        ...

def YANGListType(*args, **kwargs):
    class YANGList(object):
        ...