Added several new types for working with dates and ranges

  date_specifier_t ::
    This is like a plain date_t, except it knows what wasn't specified.
    For example, if 2008/06 is parsed, it becomes date_specifier_t which
    knows that no day was given.  If you ask for the begin() date of the
    specifier, it will be 2008/06/01; the end() date (which is
    exclusive) will be 2008/07/01.

  date_range_t ::
    A date range is a range of two specifiers, either of which (but not
    both) may be omitted.  This makes it possible to represent
    expressions like "from june to july", where no day or year is given.
    The exact dates will be inferred by using the current year, and
    fixing the range from YEAR/06/01 to YEAR/07/01.  That is, the range
    goes from the begin() of one date specifier to the begin() of the
    other.

  date_specifier_or_range_t ::
    A variadic type that can be either a date_specifier_t or a
    date_range_t.  It's just a wrapper to represent the fact that ranges
    can be implicit via specifiers (such as, "in june"), or explicit via
    ranges ("since 2008").

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