Add support for external integration/tools to react to SC events 

* Pulled (to start with) one of our events out to a separate assembly so that external endpoints can reference it
* Switched to durable storage for the subscriptions
* Added a IntegrationDemo sample to show how all this works (its just a plan nsb endpoint with a handler)

Q: I've keep the contracts project at .net4.0 to not force "integrators" to not use net4.5. YAGNI?

Please review

@SimonCropp @sfarmar @indualagarsamy @johnsimons