feat(ssl): redesign SSL usage

BREAKING CHANGE: Server::https was changed to allow any implementation
  of Ssl. Server in general was also changed. HttpConnector no longer
  uses SSL; using HttpsConnector instead.

.travis.yml

@@ -6,6 +6,8 @@ matrix:
           env: FEATURES="--features nightly"
         - rust: beta
         - rust: stable
+        - rust: stable
+          env: FEATURES="--no-default-features"
 
 sudo: false