Embedding iframes into output cells

[Pat LeSmithe]

In case there's interest... It's possible to embed any web page into an
output cell with a bit of client-side mischief:

http://www.sagenb.org/home/pub/464/

Since this uses cell_div_output_*, the embeds won't respond to requests
to hide themselves or to delete all output. Perhaps it's better to
implement this in the server.

This suggests a way to display long docstrings, live docs, or
non-standard docs inline, with scrollbars.

[Maurizio]

Nice! :)