New EmbeddableDocumentStore

- Lives in Raven.Database but has same namespace as previous EmbeddableDocumentStore to reduce number of changes. Namespace can be refactored later if desired.
- Has same signature as Raven.Client.Embedded.EmbeddableDocumentStore
- Removed (but not deleted) the Raven.Client.Embedded project from the solution.
- Behavioural changes between Embedded and full server will _probably_ result in some bugs / test failures (tough to check given that there are tests failing on New3 and it's takes hours to run them)