Update tokio to v0.2.0-alpha.1

As part of this, I made an optional tokio feature which, when enabled,
adds utility functions that spawn on the default tokio executor. This
allows for the removal of the runtime crate.

On the one hand, this makes the spawning utils slightly less generic. On
the other hand:

- The fns are just helpers and are easily rewritten by the user.
- Tokio is the clear dominant futures executor, so most people will just
  use these versions.

example-service/src/client.rs

@@ -10,7 +10,7 @@ use clap::{App, Arg};
 use std::io;
 use tarpc::{client, context};
 
-#[runtime::main(runtime_tokio::Tokio)]
+#[tokio::main]
 async fn main() -> io::Result<()> {
     let flags = App::new("Hello Client")
         .version("0.1")

example-service/src/server.rs

@@ -37,7 +37,7 @@ impl World for HelloServer {
     }
 }
 
-#[runtime::main(runtime_tokio::Tokio)]
+#[tokio::main]
 async fn main() -> io::Result<()> {
     env_logger::init();