Improve code sample in documentation, bugfix with session

Hi,

Thanks for cool framework, first of all!

I would like improve a code sample "How to add middleware via overriding run method" a little bit. The original code sample does not uses **return** keyword, which is generally considered good practices. Then, it is better to explicitly get **$app** from MiddleWare::wrap, because: a) it is not obvious to people, who don't know Plack well, does wrap call *modifies* original object or it returns new, b) it is more suitable for further Middlewares additions. 

Thanks!

WBR, basiliscos.