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

Cargo.toml

@@ -17,12 +17,12 @@ futures = "0.1.23"
 http = "0.1.10"
 hyper = "0.12.13"
 hyper-old-types = { version = "0.11", optional = true, features = ["compat"] }
-hyper-tls = "0.3"
+hyper-tls = { version = "0.3", optional = true }
 libflate = "0.1.18"
 log = "0.4"
 mime = "0.3.7"
 mime_guess = "2.0.0-alpha.6"
-native-tls = "0.2"
+native-tls = { version = "0.2", optional = true }
 serde = "1.0"
 serde_json = "1.0"
 serde_urlencoded = "0.5"
@@ -36,8 +36,9 @@ env_logger = "0.5"
 serde_derive = "1.0"
 
 [features]
-default = []
+default = ["default-tls"]
 hyper-011 = ["hyper-old-types"]
+default-tls = ["hyper-tls", "native-tls"]
 
 [package.metadata.docs.rs]
 rustdoc-args = ["--cfg", "docs_rs_workaround"]