Make SkyCoord repr so "No Frame" when a frame isn't specified

This closes #2493 if it gets merged.

This is a straightforward fix that just adjusts ``__repr__`` to say "No Frame" if a frame isn't specified at creation of a `SkyCoord`.

The downside to this is that ``sc.frame`` still yields an `ICRS` object.  Right now the canonical way to check if it's not a frame is to check if ``sc.frame_name`` is `None`.  

An alternative solution is to completely do away with `frame_name` and add a new frame class called `NoFrame` or something like that.  It wouldn't have any transforms, so it would behave the same, and ``sc.frame.name`` would no longer give a different answer from ``sc.frame_name``.  That would require more work, though - @taldcroft probably knows better if it would be a lot of work or just a little.

cc @astrofrog @taldcroft