SDSS docfixes

SDSS caused the sphinx builds to crash with errors like this:

```
ERROR: TypeError: cannot concatenate 'str' and 'type' objects [astropy.utils.misc]
Traceback (most recent call last):
  File "<stdin>", line 29, in <module>
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/application.py", line 200, in build
    self.builder.build_all()
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 159, in build_all
    self.build(None, summary='all source files', method='all')
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 216, in build
    purple, length):
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 120, in status_iterator
    for item in iterable:
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/environment.py", line 613, in update_generator
    self.read_doc(docname, app=app)
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/environment.py", line 761, in read_doc
    pub.publish()
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/docutils/core.py", line 217, in publish
    self.settings)
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/docutils/readers/__init__.py", line 71, in read
    self.input = self.source.read()
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/environment.py", line 743, in read
    app.emit('source-read', docname, arg)
  File "/Users/adam/virtual-python/lib/python2.7/site-packages/sphinx/application.py", line 314, in emit
    results.append(callback(self, *args))
  File "/Volumes/disk5/Users/adam/repos/astropy/astropy/sphinx/ext/automodapi.py", line 262, in process_automodapi
    source[0] = automodapi_replace(source[0], app, True, docname)
  File "/Volumes/disk5/Users/adam/repos/astropy/astropy/sphinx/ext/automodapi.py", line 197, in automodapi_replace
    ispkg, hascls, hasfuncs = _mod_info(modnm, toskip)
  File "/Volumes/disk5/Users/adam/repos/astropy/astropy/sphinx/ext/automodapi.py", line 247, in _mod_info
    for localnm, fqnm, obj in zip(*find_mod_objs(modname, onlylocals=True)):
  File "/Volumes/disk5/Users/adam/repos/astropy/astropy/utils/misc.py", line 188, in find_mod_objs
    fqnames.append(obj.__module__ + '.' + obj.__name__)
TypeError: cannot concatenate 'str' and 'type' objects
Sphinx Documentation subprocess failed with return code 1
```

I think this comes from not specifying ```__all__``` in the `core.py` file, so autodoc ended up looking for the wrong things....