Make `Angle` a `Quantity` subclass

As discussed in a variety of places, we should change `Angle` to be a subclass of `Quantity`.  Now that #837 is merged, the main thing `Angle` has that `Quantity` doesn't is in the initializer and some appropriate defaults for string formatting, so it's a natural subclass. 

We probably want to wait on #929 before doing this, though, as that would probably have a ripple effect into `Angle`, right @astrofrog ?