Added logic to handle Basic auth of Elasticsearch

[MedAnd]

I needed support for logging to secured Elasticsearch clusters. I added logic to handle Basic auth of Elasticsearch with the connection format as per Nest library: http://user:password@localhost:9200

Cheers, AndMed

[MedAnd]

Fix to set Auth on header only once as the client object is reused. Please ignore the previous change.

[trentmswanson]

@MedAnd did you try using the format http://user:password@domain when configuring the sink?

[MedAnd]

@trentmswanson Testing against Elasticsearch 2.1 and using http://user:password@domain did not work out of the box. I found I had to add user:password as a header which seems to have done the trick.