BUG: DateOffset weekday around DST produces unexpected results. fixes #5175

closes #5175

Wound up working on this in preparation for the DST change next week. Here's a first pass at a solution and a test.
plurals and singular are not the same -- plurals behave how you would expect, but singulars are used for fixing the time part:

    In [2]: t = pd.Timestamp('2014-06-10 8am')

    In [3]: t + pd.DateOffset(hour=9)
    Out[3]: Timestamp('2014-06-10 09:00:00', tz=None)

    In [4]: t + pd.DateOffset(hours=9)
    Out[4]: Timestamp('2014-06-10 17:00:00', tz=None)