Adding an autoclass entry in the Artist API ensures that that class
appears in the intersphinx `build/html/object.inv`, as can be checked by
```
"matplotlib.artist.Artist" in (
    sphinx.ext.intersphinx.read_inventory(
        open("build/html/objects.inv", "rb"), None, lambda *args: None)[
            "py:class"])
```
(now returns True).

This allows other projects to link to that class in their docs using
```
`Artist` <matplotlib.artist.Artist>
```

This possibility used to be present, but disappeared when the Artist API
doc got refactored.  (Other classes that appear in the API docs are fine
as they already use autoclass.)

[MRG+1] Clean up BoundaryNorm docstring

add gitter link in README

[DOC] Fix layout of spectrum_demo.py

…inx-entry

Add autoclass entry for Artist API doc.

Modified axhline() function to save state of ignoring limits and restore after plotting the line