Added marathon API access token key support #6 #10

[pawmart]

Marathon API token addition resolved all my problems.

Posting new cert to marathon-lb
Retrieving current marathon-lb cert
Comparing old cert to new cert
Deploying marathon-lb with new cert
Waiting for deployment to complete
Waiting for deployment to complete
Deployment complete

[mattj-io]

We are moving this repo to https://github.com/dcos-labs/letsencrypt-dcos, and I'd like to get authorization options into the code base.