Change argument order in AngularSeparation from (lat, lon) to (lon, lat)

At the moment the argument order of the AngularSeparation constructor is `lat` first:
```
astropy.coordinates.AngularSeparation(self, lat1, lon1, lat2, lon2, units)
```

As far as I can tell everywhere else in astropy (and in most other software) the order is `lon` first.
```
astropy.coordinates.angle_utilities.simple_sphere_dist(lon1, lat1, lon2, lat2)
In [26]: astropy.coordinates.GalacticCoordinates(1, 2, unit=('deg', 'deg'))
Out[26]: <GalacticCoordinates l=1.00000 deg, b=2.00000 deg>
```

I think it would be worth changing the argument order to `lon` first to be consistent.

It's a trivial change in the `__init__` and class docstring (see [here](https://github.com/astropy/astropy/blob/master/astropy/coordinates/angles.py#L654)) ... I didn't make a pull request because I don't know what the deprecation process is in astropy at the moment.