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.

src/lib.rs

@@ -25,6 +25,8 @@
 //! - `runtime` (*enabled by default*): Enables convenient integration with
 //!   `tokio`, providing connectors and acceptors for TCP, and a default
 //!   executor.
+//! - `tcp` (*enabled by default*): Enables convenient implementations over
+//!   TCP (using tokio).
 //! - `unstable-stream` (*unstable*): Provides `futures::Stream` capabilities.
 //!
 //!   Due to the `Stream` trait not being stable, this feature is also