Provide optional features on tokio crate (#808)

Disabling all features means the only dependency is `futures`.

Relevant pieces of the API can then be enabled with the following features:

- `codec`
- `fs`
- `io`
- `reactor`
- `tcp`
- `timer`
- `udp`
- `uds`

This also introduces the beginnings of enabling only certain pieces of the `Runtime`. As a start, the entire default runtime API is enabled via the `rt-full` feature.

src/net.rs

@@ -22,6 +22,7 @@
 //! [`UnixDatagram`]: struct.UnixDatagram.html
 //! [`UnixDatagramFramed`]: struct.UnixDatagramFramed.html
 
+#[cfg(feature = "tcp")]
 pub mod tcp {
     //! TCP bindings for `tokio`.
     //!
@@ -42,15 +43,19 @@ pub mod tcp {
     //! [`Incoming`]: struct.Incoming.html
     pub use tokio_tcp::{ConnectFuture, Incoming, TcpListener, TcpStream};
 }
+#[cfg(feature = "tcp")]
 pub use self::tcp::{TcpListener, TcpStream};
 
+#[cfg(feature = "tcp")]
 #[deprecated(note = "use `tokio::net::tcp::ConnectFuture` instead")]
 #[doc(hidden)]
 pub type ConnectFuture = self::tcp::ConnectFuture;
+#[cfg(feature = "tcp")]
 #[deprecated(note = "use `tokio::net::tcp::Incoming` instead")]
 #[doc(hidden)]
 pub type Incoming = self::tcp::Incoming;
 
+#[cfg(feature = "udp")]
 pub mod udp {
     //! UDP bindings for `tokio`.
     //!
@@ -68,16 +73,19 @@ pub mod udp {
     //! [`framed`]: struct.UdpSocket.html#method.framed
     pub use tokio_udp::{RecvDgram, SendDgram, UdpFramed, UdpSocket};
 }
+#[cfg(feature = "udp")]
 pub use self::udp::{UdpFramed, UdpSocket};
 
+#[cfg(feature = "udp")]
 #[deprecated(note = "use `tokio::net::udp::RecvDgram` instead")]
 #[doc(hidden)]
 pub type RecvDgram<T> = self::udp::RecvDgram<T>;
+#[cfg(feature = "udp")]
 #[deprecated(note = "use `tokio::net::udp::SendDgram` instead")]
 #[doc(hidden)]
 pub type SendDgram<T> = self::udp::SendDgram<T>;
 
-#[cfg(unix)]
+#[cfg(all(unix, feature = "uds"))]
 pub mod unix {
     //! Unix domain socket bindings for `tokio` (only available on unix systems).
 
@@ -86,5 +94,5 @@ pub mod unix {
         UnixListener, UnixStream,
     };
 }
-#[cfg(unix)]
+#[cfg(all(unix, feature = "uds"))]
 pub use self::unix::{UnixDatagram, UnixDatagramFramed, UnixListener, UnixStream};