make `temporenc.Value` hashable about temporenc-python

[wbolster]

The datetime classes hash to the same value if their constituent parts are the same:

>>> d1 = datetime.date(2014, 10, 23)
>>> d2 = datetime.date(2014, 10, 23)
>>> {d1: 1, d2: 2}
{datetime.date(2014, 10, 23): 2}
>>> d1 is d2
False
>>> d1 == d2
True