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
@gmarz we could also pass a `IDictionary<string, object>` instead of the anonymous object but `JsonFormatter` seems to be way to go looking at other sinks. 

Stylistically `if (items.Any() && _client != null)` could be an early exit at the top of the method, and instead of `ToList()` items and create a seperate `bulk` list use `.Select()` and pass that `IEnumerable<string>` to `Bulk()`