Add TimePlotDate format for input to matplotlib plot_date()

Matplotlib date input: 1 + number of days from 0001-01-01 00:00:00 UTC

This can be used directly in the matplotlib `plot_date` function::

    >>> jyear = np.linspace(2000, 2001, 20)
    >>> t = Time(jyear, format='jyear', scale='utc')
    >>> plt.plot_date(t.plot_date, jyear)
    >>> plt.gcf().autofmt_xdate()  # orient date labels at a slant
    >>> plt.draw()