reduce dependency tree by features

Cargo.toml

@@ -9,15 +9,21 @@ readme = "README.md"
 keywords = ["web", "request", "http", "rest", "client"]
 categories = ["web-programming::http-client"]
 
+[features]
+default = ["tls"]
+json = ["serde_json"]
+charset = ["encoding"]
+tls = ["native-tls"]
+
 [dependencies]
 ascii = "0.9"
 base64 = "0.9"
 chunked_transfer = "0.3"
 cookie = { version = "0.10", features = ["percent-encode"] }
 dns-lookup = "0.9.1"
-encoding = "0.2"
 lazy_static = "1"
 qstring = "0.6"
-native-tls = "0.1"
-serde_json = "1"
 url = "1.6"
+native-tls = { version = "0.1", optional = true }
+serde_json = { version = "1", optional = true }
+encoding = { version = "0.2", optional = true }