Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
I disagree. At the moment, the context is configured by an express middleware. It implies that the `http` transport is in use. It's reasonably to set up the initial context under the `http` property. Why do we need to create two middleware to inject the `http` context?

I understand your desire to have clean separations. But adding transport-level information is not polluting the context. strong-remoting is bridge between transport and business logic. It's often desirable to have access to the transport-level context. Sure, in the ideal world, we can populate all useful data into the context by different middleware, but it's not possible to cover all cases. Having access to the transport context is very similar as exposing the driver from the ORM.