RavenDB sink

An implementation of a sink for storing logevents to RavenDB. It
inherits from PeriodicBatchingSink and uses RavenDB's async session.
The current implementation is using a wrapper class around the logevent
(like the azuretablestorage sink) to store as a document.
There's 1 unit test that uses the embeddable documentStore to store and query the logevents.