add a Proxy type

Proxies can currently be configured to intercept HTTP, HTTPS, and all
requests. HTTPS tunneling is supported.

Closes #30

Cargo.toml

@@ -13,7 +13,7 @@ categories = ["web-programming::http-client"]
 bytes = "0.4"
 futures = "0.1.14"
 hyper = "0.11"
-hyper-tls = "0.1"
+hyper-tls = "0.1.1"
 libflate = "0.1.5"
 log = "0.3"
 native-tls = "0.1"
@@ -21,6 +21,7 @@ serde = "1.0"
 serde_json = "1.0"
 serde_urlencoded = "0.5"
 tokio-core = "0.1.6"
+tokio-io = "0.1"
 url = "1.2"
 
 [dev-dependencies]