Added RavenDB sink

An implementation of a sink for storing logevents to RavenDB. It
implements 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.
Merge branch 'master' of https://github.com/nickvane/serilog into nickvane-master
Make spelling of timestamp in RavenDB sink consistent with LogEvent.
Getting to know the contributed RavenDB sink by tinkering with the test case.