Fix dms hms attr signs

As pointed out by @ejesche on astropy-dev, the behavior of the ``.hms`` and ``.dms`` attributes changed from v0.2 such that they now give e.g. ``(-h,m,s)`` instead of ``(-h,-m,-s)`` when the angle is negative.  I've traced this change to #1006, and I *think* this was accidental, as its part of fixing the functions to work correctly with arrays, rather than following a specific discussion on the topic. So this PR changed it so that v0.3 is consistent with v0.2, and updates the tests to match.

I know there have been a few conversations on this topic, so there may have been a shift in favor of ``(-h,-m,-s)``.  If so, I think we're better off doing it in the *next* version, when the coordinates API is changing a lot, anyway.