Prevent double check-in and check-out: an example of domain logic via event-sourced aggregates

[Ocramius]

This patch implements a feature (that will be specified later on, for the sake of the simplicity of the example) that prevents double check-in and checking out when not checked in.

To be kept in mind:

1. multiple past UserCheckedIn and UserCheckedOut are possible, since the business rule is only enforced once this patch is merged, hence past state may be inconsistent with new rules. This is exactly like in typical legal systems: laws are not retroactive.
2. multiple past UserCheckedIn and UserCheckedOut must therefore be either ignored or handled gracefully, so that future interactions do not cause awkwardness (if somebody checked in twice, for example, they shouldn't be required to check out twice, but just once)