count() doesn't work properly using UNION
Thank you, fixed.

Hello,

following code currently doesn't work properly.

```
q = (q1 | q2)
print(q.count())
```

I tested to modify peewee.py as follows.

```
    def count(self, clear_limit=False):
        if self._distinct or self._group_by or self._limit or self._offset or \
                isinstance(self, CompoundSelect):
            return self.wrapped_count(clear_limit=clear_limit)
```

And then it went well.
Please confirm whether this is right.
