Support for request and session-scoped instances in intravenous

E.g. consider you authorize requests and have user instance which describes a current user during the request. It would be nice to have an option to store it in container and reuse in other services during a request. 

The idea is taken from CDI in Java.

But it is really nice to have or at least I think it is good idea to make it possible to extend intravenous with functionality like this. 
