Aggregation registration. Fixes #13

[jonclare]

Enhance and extend the ApplyEventRouterconcept to make it more full-featured.

• Externalise the registration from aggregates themselves
• Extend the registry to contain routes for multiple aggregates
• Add ability to register and dispatch commands from the registry
• Move EventNameMapto the external registry
• Add a container for aggregate metadata in the registry that can be used to determine keysd
• Add beginnings of snapshotting via ISnapshotRepository and EventStoreSnapshotRepository
• Introduce the concept of versions for aggregates separate to the domain.
• Add a layer that wraps the events and snapshot repositories to allow consumers to deal with aggregates and versions rather than simply sequences of events