Rewrite relativedelta.__add__ based on dateutil 2.4.0
* completely remove any hint of supporting time deltas (could be
implemented in the future, but not currently asked for and lots of
code to implement)
* implement weekday support (ordinal only)
* probably better leapyear support
* remove relativedelta.__sub__ (only works between relativedeltas)
* add relativedelta.__neg__
fixes#5089
Xavier Morel
fb2b1d7568