Allow https communication per ec2 or s3 service

By default all communication w/ AWS services done by this plugin is sent the clear over `http`, overriding amazons own default of https: http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/ClientConfiguration.html#getProtocol()

One has to set `cloud.aws.protocol` in `elasticsearch.yml` to force SSL.

    cloud.aws.protocol: https

This is not entirely clear to the average user, and should be added to the documentation on both this project's README.

Closes #101.
Switch to https communication for Amazon APIs by default

We should use `https` by default instead of `http` for communication between elasticsearch and AWS API.

Note that it can be modified in case of trouble and fallback to the older setting using `cloud.aws.protocol: http`

Closes #109.