* Fix a few remaining typos
* Wrap lines to 80 chars
* Fix the conversion from Textile to Markdown
* Update the part about `Rack::Cache` which isn't included anymore and
speak about Active Support's provided middleware
* Remove references to out-of-date methods and middlewares (e.g.
respond_with) and update the list of modules and middlewares to match
the actual code-base.
* Remove the middleware's description and links to the Rack guide
