Owin embedded host attempt2

 - RavenDbServer has been moved to Raven.Database and will effectively be the replacement for EmbeddableDocumentStore.
 - The owin embedded code is a mixture of stuff from KatanaProject's MS.Owin.Testing and my own Owin.HttpMessageHandler (Apache2 and MIT respectively). The code style matches these projects, easier to track diffs / fixes / etc and can apply raven's style when it matures out a bit.
 - HttpJsonRequest\Factory has been modified to allow specification of an HttpMessageHandler ( OwinClientHandler ).
 - Raven.Database references Client.Lightweight and exposes the "embedded" client via RavenDbServer.DocumentStore property.