Adding Elasticsearch HTTP authentication functionality for issue #262

Hello,

This relates to https://github.com/MeetMe/newrelic-plugin-agent/issues/262 .  I needed to get this working for my Elasticsearch/NewRelic install, so I've checked that this works like it ought to.  There may be a more elegant way to code this - apologies, I'm not a Python programmer, but I think this (or something like it) ought to be pretty uncontroversial.

Thanks!