scl/elasticsearch: Add a simple, python-based ES destination

This adds an elasticsearch() destination, that uses the Python Requests
library to send JSON-formatted data to Elasticsearch. It is lacking
error handling, and isn't a speed daemon, either, but it introduces a
destination config API, which we can keep, when the underlying code
changes.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
README.md: Mention that this branch does not work with 3.6

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>