TypeError when creating a pidfile

Hello!

I've discovered an issue when using gunicorn 0.17.2 on Python 3.3.0 with the following configuration settings:

`daemon = True`
`pidfile = '/some/path/...'`

Here's the stack trace:

    Traceback (most recent call last):
      File "/srv/users/kevin/apps/five/venv/bin/gunicorn", line 9, in <module>
        load_entry_point('gunicorn==0.17.2', 'console_scripts', 'gunicorn')()
      File "/srv/users/kevin/apps/five/venv/lib/python3.3/site-packages/gunicorn/app/wsgiapp.py", line 34, in run
        WSGIApplication("%(prog)s [OPTIONS] APP_MODULE").run()
      File "/srv/users/kevin/apps/five/venv/lib/python3.3/site-packages/gunicorn/app/base.py", line 131, in run
        Arbiter(self).run()
      File "/srv/users/kevin/apps/five/venv/lib/python3.3/site-packages/gunicorn/arbiter.py", line 170, in run
        self.start()
      File "/srv/users/kevin/apps/five/venv/lib/python3.3/site-packages/gunicorn/arbiter.py", line 128, in start
        self.pidfile.create(self.pid)
      File "/srv/users/kevin/apps/five/venv/lib/python3.3/site-packages/gunicorn/pidfile.py", line 39, in create
        os.write(fd, "%s\n" % self.pid)
    TypeError: 'str' does not support the buffer interface

Changing line 39 to `os.write(fd, ("%s\n" % self.pid).encode('utf-8'))` has solved the problem for me without any obvious errors in the last few days.