Fix next_date calculation

Fixes #6. next_date attempted to calculate the date of a given weekday
in a given week of the month, instead of the date of a particular
instance of a given weekday of a given month. This assumption fell down
when attempting to find the first incident of a weekday in a month that
started on a later day of the week. For example:

    >>> calendar.monthcalendar(2014, 11)[0]
    [0, 0, 0, 0, 0, 1, 2]
    >>> calendar.monthcalendar(2014, 11)[0][3]
    0