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.

src/reactor/global.rs

@@ -0,0 +1,43 @@
+use std::io;
+use std::thread;
+
+use reactor::{Reactor, Handle};
+
+pub struct HelperThread {
+    thread: Option<thread::JoinHandle<()>>,
+    reactor: Handle,
+}
+
+impl HelperThread {
+    pub fn new() -> io::Result<HelperThread> {
+        let reactor = Reactor::new()?;
+        let reactor_handle = reactor.handle().clone();
+        let thread = thread::Builder::new().spawn(move || run(reactor))?;
+
+        Ok(HelperThread {
+            thread: Some(thread),
+            reactor: reactor_handle,
+        })
+    }
+
+    pub fn handle(&self) -> &Handle {
+        &self.reactor
+    }
+
+    pub fn forget(mut self) {
+        drop(self.thread.take());
+    }
+}
+
+impl Drop for HelperThread {
+    fn drop(&mut self) {
+        // TODO: kill the reactor thread and wait for it to exit, needs
+        //       `Handle::wakeup` to be implemented in a future PR
+    }
+}
+
+fn run(mut reactor: Reactor) {
+    loop {
+        reactor.turn(None);
+    }
+}