Promote InMemoryEventStore to the main Eventuous library

[mvastarelli]

The Eventous.TestHelpers project defines an in-memory store for persisting events. It would be helpful if this was offered as one of the available stores within the main Eventuous library since developers could utilize it for both local development and testing without having to either roll their own or stand up a local database.

For comparison, EventFlow defines in-memory as one of their available stores (with the caveat that it's for testing only) and will actually fall back on it by default if no store is configured.