feat(lib): add optional tcp feature, split from runtime

The `HttpConnector` and `AddrListener` types which make use of
`tokio::tcp` have been made their own optional feature. This allows
using them without requiring the *full* tokio runtime.

Cargo.toml

@@ -27,7 +27,6 @@ futures-util-preview = { version = "=0.3.0-alpha.19" }
 http = "0.1.15"
 http-body = "=0.2.0-alpha.1"
 httparse = "1.0"
-# h2 = "=0.2.0-alpha.2"
 h2 = "=0.2.0-alpha.3"
 iovec = "0.1"
 itoa = "0.4.1"
@@ -35,15 +34,19 @@ log = "0.4"
 net2 = { version = "0.2.32", optional = true }
 pin-project = "0.4"
 time = "0.1"
-tokio = { version = "=0.2.0-alpha.6", optional = true, default-features = false, features = ["rt-full"] }
 tower-service = "=0.3.0-alpha.2"
 tower-make = { version = "=0.3.0-alpha.2a", features = ['io'] }
-tokio-executor = { version = "=0.2.0-alpha.6", features = ["blocking"] }
+tokio-executor = "=0.2.0-alpha.6"
 tokio-io = "=0.2.0-alpha.6"
 tokio-sync = "=0.2.0-alpha.6"
+want = "0.3"
+
+# Optional
+
+tokio = { version = "=0.2.0-alpha.6", optional = true, default-features = false, features = ["rt-full"] }
 tokio-net = { version = "=0.2.0-alpha.6", optional = true, features = ["tcp"] }
 tokio-timer = { version = "=0.3.0-alpha.6", optional = true }
-want = "0.3"
+
 
 [dev-dependencies]
 matches = "0.1"
@@ -64,8 +67,12 @@ default = [
     "runtime",
 ]
 runtime = [
-    "net2",
+    "tcp",
     "tokio",
+]
+tcp = [
+    "net2",
+    "tokio-executor/blocking",
     "tokio-net",
     "tokio-timer",
 ]