put all TLS functionality behind a feature flag

The "Cargo feature" `default-tls`, which is enabled by default, is
added, with all TLS support relying on it. This allows using reqwest but
disabling the `native-tls` dependency, by disabling this feature.

Closes #225

.travis.yml

@@ -8,18 +8,15 @@ matrix:
           rust: stable
 
         - rust: stable
-          env: FEATURES=""
         - rust: beta
-          env: FEATURES=""
         - rust: nightly
-          env: FEATURES=""
+
+        # Disable default-tls
+        - rust: stable
+          env: FEATURES="--no-default-features"
 
         - rust: stable
           env: FEATURES="--features hyper-011"
-        - rust: beta
-          env: FEATURES="--features hyper-011"
-        - rust: nightly
-          env: FEATURES="--features hyper-011"
 
         # minimum version
         - rust: 1.26.0