Midware dependency detection / support

[despeset]

We have some sketched in fields on our Midware type for supporting dependency detection and analysis.

immutable Midware
    handler::Function
    expects::Array
    provides::Array
end

I think the general idea here is just to provide a method of giving developers better error messages when they try to build a stack that does something like put Sessions before CookieParser... We need to decide exactly how these are specified so the expects / provides interface doesn't cause more trouble than it's worth ( like "cookies" vs. "Cookies" ).