API: make Series.sort/order arguments in the same order
- drop unused `Series.sort` order argument
- added `Series.sort na_position` argument to conform with `Series.order`
- default kind for `Series.order` is now `quicksort`, same as `Series.sort` and numpy default

closes #6847

cc @unutbu 
 any objections here?

@jtratner @cpcloud @jorisvandenbossche @hayd 
 fine by me 
 :+1: 
 Are `sort` and `order` now fully equivalent? (apart from that `sort` is inplace and `order`not?) Maybe this should be added to the docstrings (eg a "See also")
 yes, good point
 fixed in https://github.com/pydata/pandas/commit/c1440e9b1b206b9c892105ce68f3b88f2436f5a5

as an aside, maybe we should add an `inplace` argument to `sort=True` (so if False it will return `order`)?
 And then also in `order` the other way around with `inplace=False`?
 I think that is right....seems innocuous enough (famous last words)...will make an issue
