Incorrect SQL params in order_by expression
Thanks!

```
Player.select().order_by(Player.elo * 5)
```

printing the SQL:

```
('SELECT t1."id", t1."created_at", t1."last_modified", t1."wins_t", t1."wins_z", t1."wins_p", t1."losses_t", t1."losses_z", t1."losses_p", t1."username", t1."race", t1."elo" FROM "player" AS t1 ORDER BY (t1."elo" * ?)', [])
```

error:

```
Traceback (most recent call last):
  File "manage.py", line 43, in <module>
    func(sys.argv[2:])
  File "manage.py", line 25, in cmd_test
    for k in Player.winrate_order():
  File "/Users/jonanin/dev/sc2site/env/lib/python2.7/site-packages/peewee.py", line 1563, in __iter__
    return iter(self.execute())
  File "/Users/jonanin/dev/sc2site/env/lib/python2.7/site-packages/peewee.py", line 1556, in execute
    self._qr = ResultWrapper(self.model_class, self._execute(), query_meta)
  File "/Users/jonanin/dev/sc2site/env/lib/python2.7/site-packages/peewee.py", line 1314, in _execute
    return self.database.execute_sql(sql, params, self.require_commit)
  File "/Users/jonanin/dev/sc2site/env/lib/python2.7/site-packages/peewee.py", line 1721, in execute_sql
    res = cursor.execute(sql, params or ())
sqlite3.ProgrammingError: Incorrect number of bindings supplied. The current statement uses 1, and there are 0 supplied.
```
