Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
As I said before, I am opposed to exposing the http request & response objects to shared methods. Instead, there should be an easy and well-documented way how to write an express middleware that takes the relevant information from the request and set it on the context.

The new example code should look like this:

```js
CartItem.sum = function(cartId, callback) {
  // ...
  var ns = loopback.getCurrentContext();
  console.log(ns.get('requestUrl'));
};

// elsewhere - add a middleware to fill the context
app.use(function(req, res, next) {
  var ns = loopback.getCurrentContext();
  ns.set('requestUrl', req.url);
  next();
});