Start adding a global event loop

This commit starts to add support for a global event loop by adding a
`Handle::default` method and implementing it. Currently the support is quite
rudimentary and doesn't support features such as shutdown, overriding the return
value of `Handle::default`, etc. Those will come as future commits.

tests/global.rs

@@ -0,0 +1,37 @@
+extern crate futures;
+extern crate tokio;
+
+use std::thread;
+
+use futures::prelude::*;
+use tokio::net::{TcpStream, TcpListener};
+use tokio::reactor::Handle;
+
+macro_rules! t {
+    ($e:expr) => (match $e {
+        Ok(e) => e,
+        Err(e) => panic!("{} failed with {:?}", stringify!($e), e),
+    })
+}
+
+#[test]
+fn hammer() {
+    let threads = (0..10).map(|_| {
+        thread::spawn(|| {
+            let handle = Handle::default();
+            let srv = t!(TcpListener::bind(&"127.0.0.1:0".parse().unwrap(), &handle));
+            let addr = t!(srv.local_addr());
+            let mine = TcpStream::connect(&addr, &handle);
+            let theirs = srv.incoming().into_future()
+                .map(|(s, _)| s.unwrap().0)
+                .map_err(|(s, _)| s);
+            let (mine, theirs) = t!(mine.join(theirs).wait());
+
+            assert_eq!(t!(mine.local_addr()), t!(theirs.peer_addr()));
+            assert_eq!(t!(theirs.local_addr()), t!(mine.peer_addr()));
+        })
+    }).collect::<Vec<_>>();
+    for thread in threads {
+        thread.join().unwrap();
+    }
+}