Add context propagation middleware

 - Implement the middleware `loopback.context`
 - Inject context into juggler and strong-remoting
 - Make http context optional and default to false
 - Optionally mount context middleware from `loopback.rest`
Enable the context middleware from loopback.rest
Move `context` example to a standalone app
rest middleware: clean up context config

Modify `loopback.rest()` to read the configuration for
`loopback.context` from `app.get('remoting')`, which is the approach
used for all other configuration options related to the REST transport.
Merge pull request #337 from strongloop/feature/context-propagation

Add context propagation middleware