Include unit conversion members only on subclasses of `Quantity`

In #1006, it was suggested by @astrofrog that the implicit conversions to other units, e.g.:

```
(5.0 * u.m).pc
```

should only be included on subclasses of `Quantity`, not `Quantity` itself.  I'm not sure I agree -- I think it's very handy and useful.

It's an easy implementation, but I thought we should discuss first.

(Note that this functionality is not in 0.2.x, so we don't have any backward compatibility to worry about).