Fix return value of Write::write impl for Pipe (#120)

* Fix return value of `Write::write` impl for `Pipe`

This should return the number of bytes written, which as implemented
is the whole buffer.

Without this, the default `write_all` implementation won't work
correctly, which is a problem with
c520a21.

* Use `.write` instead of `printfl!` to write `""`

Now that `printfl!` calls `write_all`, rather than `write`, this no
longer works. The `write_all` implementation provided by the `Write`
trait writes until the buffer of bytes left to write is empty, so `write`
is never if an empty string is passed, and thus a `Pipe` doesn't send a
message with `done`.

There are probably better ways to deal with the APIs involved here, but
this at least fixes the behavior.

src/multi.rs

@@ -251,7 +251,7 @@ impl Write for Pipe {
                 string: s,
             })
             .unwrap();
-        Ok(1)
+        Ok(buf.len())
     }
 
     fn flush(&mut self) -> Result<()> {

src/pb.rs

@@ -429,7 +429,7 @@ impl<T: Write> ProgressBar<T> {
     /// the last time
     pub fn finish(&mut self) {
         self.finish_draw();
-        printfl!(self.handle, "");
+        self.handle.write(b"").expect("write() failed");
     }
 
     /// Call finish and write string `s` that will replace the progress bar.