! can: move `client.ssl-encryption` setting from reference.conf into `Http.Connect` message, fixes #396

This has several implications:

 * user-level API is smaller now
 * there usually will be only one pipeline for all connections (because
   there will be only one distinct settingsGroup in the HttpManager)
 * we will throw an error if we find a user using the old
   `spray.can.client.ssl-encryption` setting to make sure they don't rely on
   it