Add inherit_middleware class accessor, allowing users to turn off middleware inheritance

If you are using a design where you have a parent Roda class as the
main rack app, and having the parent class dispatching to subclasses
via run, you don't want the middlware inherited, as it will result
in the middleware processing the requests multiple times.
Add clear_middleware! class method, for clearing the current middleware

This can be used in cases where middleware are inherited from a
parent class by default, but you want to clear them for a specific
subclass.
Add links to some applications using Roda to the website
Add drop_body plugin, for automatically dropping body and Content-{Length,Type} headers based on response status