Use Elasticsearch.Net instead of NEST

Hey guys, love the library!

I'd like to propose a few improvements to the Elasticsearch sink.

Since the sink is write-only, it doesn't benefit from the added overhead of NEST and additional dependency on Json.NET.  Elasticsearch.Net is more light-weight and better suited for Serilog's use case.  This PR removes the NEST and Json.NET dependencies and replaces them with Elasticsearch.Net.

Additionally, I added two additional extension methods to provide more control over the connection to Elasticsearch:

1) A convenience method that accepts a collection of URIs (as opposed to a single URI) and creates a static connection pool to ES, providing cluster failover support.

2) A method that just takes an Elasticsearch.Net `ConnectionConfiguration` to give the user full control over all of the available connection options.

cc @mivano and @Mpdreamz
Yea, this is the bit I don't like.  I would have liked to use an anonymous object here and let Elasticsearch.Net serialize it, but the problem was with the `@` in the  `@timestamp` field.  Since it's a reserved C# token, it's ignored, and serializes to just `timestamp` which doesn't work for Logstash.

I wasn't aware of the `JsonFormatter` in Serilog.  I'll take a look.