add render_description method to RSSItemView to improve modularity. The construction of the description marker of and rss entry is currently built by the main cell_call function. This make the job hard for RSSItemView subclass to change the default behaviour. The changeset fix this issue by extracting the description creation into a render_description method. Such extraction are already in place for various rss marquer.

\.svn
^build$
^dist$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$