Elastic Search sink

I also added a sink to log into ElasticSearch. By default it is compatible with the logstash format, so you can just hook it up to Kibana. It uses the PeriodicBatchingSink and the NEST library to connect to ES. Just like RavenDB and MongoDB, ElasticSearch is a great NoSQL store for a logging system as Serilog.

Hope you can add this to Serilog or give me some feedback if not. Thanks!