Fix Angle initialization for lists of quantities

This PR fixes Angle (and its subclasses) to be initialized in the following manner:
```
>>> coordinates.Angle([2*u.deg, 1*u.hourangle])
<Angle [  2., 15.] deg>
```

That is, you can now give it a list of quantities and it will convert them to a single quantity.  Note that this is already how `Quantity` works - this just fixes `Angle` to more closely match `Quantity`.

Having this behavior work will make part of the APE5 implementation quite a bit easier. 

cc @mdboom @mhvk