Add function to get unified I/O formats and auto-update read/write docstrings

This PR does a few things:

- Add `io.registry.get_formats(data_class=None)` function to get a Table of the available formats for `data_class`.
- Add code in `register_reader` and `register_writer` to automatically insert an RST table of the available formats into the docstring of the appropriate class method.
- Include the table of available formats into the exception message if the user gives an invalid format.

See the preview [Table.write](http://hea-www.harvard.edu/~aldcroft/tmp/astropy/html/_generated/astropy.table.table.Table.html#astropy.table.table.Table.write) docstring (and the corresponding Table.read a little up the page) to see how this looks.  The table values there are automatically created.

An example of the new exception message is:
```
Exception: No writer is defined for format 'jun' and class 'Table'.
The available formats are:
           Format           Read Write Auto-identify
--------------------------- ---- ----- -------------
                     aastex  Yes   Yes            No
                      ascii  Yes   Yes            No
               ascii.aastex  Yes   Yes            No
                ascii.basic  Yes   Yes            No
     ascii.commented_header  Yes   Yes            No
          ascii.fixed_width  Yes   Yes            No
ascii.fixed_width_no_header  Yes   Yes            No
 ascii.fixed_width_two_line  Yes   Yes            No
                 ascii.ipac  Yes   Yes            No
                ascii.latex  Yes   Yes           Yes
            ascii.no_header  Yes   Yes            No
                  ascii.rdb  Yes   Yes           Yes
                  ascii.tab  Yes   Yes            No
                       fits  Yes   Yes           Yes
                       hdf5  Yes   Yes           Yes
                       ipac  Yes   Yes            No
                      latex  Yes   Yes            No
                        rdb  Yes   Yes            No
                    votable  Yes   Yes           Yes
```

Although I expect some of this to get re-worked when the unified I/O structured is refactored to use a class structure, for now this should go a long way to improving usability.