Fix locations / links in astropy.modeling Sphinx API docs

As mentioned in #1826, there is still a problem with locations of things in the Sphinx API docs for (at least some) sub-packages that have an extra layer in the hierarchy.
E.g.  [`astropy.modeling.functional_models.Gaussian1D`](http://docs.astropy.org/en/latest/api/astropy.modeling.functional_models.Gaussian1D.html)
should show up at `astropy.modeling.models.Gaussian1D` and I think e.g. [`astropy.io.votable.tree.VOTableFile`](http://docs.astropy.org/en/latest/api/astropy.io.votable.tree.VOTableFile.html) should show up at `astropy.io.votable.VOTableFile`.

This is a first attempt at putting everything in the appropriate place (i.e. where the user is supposed to import from) for `astropy.modeling`.

You can see the resulting html docs [here](http://www.mpi-hd.mpg.de/personalhomes/deil/public/modeling_api_sphinx/modeling/index.html#reference-api) ... it doesn't work, i.e. there are no entries under `astropy.modeling.models`.

@kbarbary @astrofrog @eteq Why is `.. automodapi:: astropy.modeling.models` not picking up the `astropy.modeling.models.__all__` entries?

```
$ python -c 'import astropy.modeling as m; print(m.models.__all__)'
[u'Pix2Sky_AZP', u'Sky2Pix_AZP', u'Pix2Sky_CAR', u'Sky2Pix_CAR', u'Pix2Sky_CEA', u'Sky2Pix_CEA', u'Pix2Sky_CYP', u'Sky2Pix_CYP', u'Pix2Sky_MER', u'Sky2Pix_MER', u'Pix2Sky_SIN', u'Sky2Pix_SIN', u'Pix2Sky_STG', u'Sky2Pix_STG', u'Pix2Sky_TAN', u'Sky2Pix_TAN', u'RotateCelestial2Native', u'RotateNative2Celestial', u'MatrixRotation2D', u'Chebyshev1D', u'Chebyshev2D', u'InverseSIP', u'Legendre1D', u'Legendre2D', u'Polynomial1D', u'Polynomial2D', u'SIP', u'OrthoPolynomialBase', u'PolynomialModel', u'AiryDisk2D', u'Beta1D', u'Beta2D', u'Box1D', u'Box2D', u'Const1D', u'Const2D', u'Disk2D', u'Gaussian1D', u'Gaussian2D', u'Linear1D', u'Lorentz1D', u'MexicanHat1D', u'MexicanHat2D', u'Ring2D', u'Scale', u'Shift', u'Sine1D', u'Trapezoid1D', u'TrapezoidDisk2D', u'custom_model_1d', u'BrokenPowerLaw1D', u'ExponentialCutoffPowerLaw1D', u'LogParabola1D', u'PowerLaw1D']
```