add app-context properties to Request and Response

As we talked about in IRC.

Provide properties for application developers to use for app-specific context data that doesn't fit well into req.env or resp.{body,data}.

I'm not sure if pre-assigning resp.context to a dictionary is the best thing.  It might be better to allow it to be None and define it as an app-specified type, or at least allow some sort of API-object-level config so developers can pick the dictionary-like class of their choice to avoid an unnecessary re-assignment if they want to override it (something I would have to do, since I plan to use this to hold Resource class).

Anyway, we've mentioned the need for more conversation about this in IRC, so I guess we should start with the lowest common denominator feature for now.