Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
IMHO the context propagation should be added automatically by `loopback()` as the first middleware in the chain.

That way the minimal loopback app still keeps working:

```js
var app = loopback();
app.use(loopback.rest());
app.listen();
```

What are the situations when a developer will not want to use `loopback.context()`? My understanding is that features like transactions will require the context , thus it is required to include the context middleware anyways.