MAINT (API?): organize npyio.recfromcsv defaults

Organizes the default kwargs in recfromcsv. Changes two undocumented
kwargs behaviors:

*  previously, if a user set `names=None`, it was ignored and replaced
   with `names=True`
*  the `dtype` kwarg was ignored. If `update` was given, it was used as
   `dtype`, and if not, None was used. We can retain the `update` behavior
 by using `kwargs.setdefault("dtype",kwargs.get('update', None))`.
  This Closes #311 .
TST: Added regression test for recfromcsv dtype error, as suggested by @hpaulj
DOC: npyio.recfromcsv keyword argument changes

Added a note to recfromcsv about the `dtype` keyword,
as suggested by @hpaulj. Also added a note to the release notes,
about the change in the `update` keyword, as suggested by @charris.