Rename to tokio-core, add in futures-io

Renames the futures-mio crate to tokio-core, pulls in the futures-io crate under
an `io` module, and gets everything compiling.

src/io/flush.rs

@@ -0,0 +1,39 @@
+use std::io::{self, Write};
+
+use futures::{Poll, Future};
+
+/// A future used to fully flush an I/O object.
+///
+/// Resolves to the underlying I/O object once the flush operation is complete.
+///
+/// Created by the `flush` function.
+pub struct Flush<A> {
+    a: Option<A>,
+}
+
+/// Creates a future which will entirely flush an I/O object and then yield the
+/// object itself.
+///
+/// This function will consume the object provided if an error happens, and
+/// otherwise it will repeatedly call `flush` until it sees `Ok(())`, scheduling
+/// a retry if `WouldBlock` is seen along the way.
+pub fn flush<A>(a: A) -> Flush<A>
+    where A: Write,
+{
+    Flush {
+        a: Some(a),
+    }
+}
+
+impl<A> Future for Flush<A>
+    where A: Write,
+{
+    type Item = A;
+    type Error = io::Error;
+
+    fn poll(&mut self) -> Poll<A, io::Error> {
+        try_nb!(self.a.as_mut().unwrap().flush());
+        Poll::Ok(self.a.take().unwrap())
+    }
+}
+