Database.drop_tables hide warning when a table doesn't exist
When I test this locally it appears to be working correctly:

``` python
In [4]: db.drop_tables([User])

.... traceback ....

/home/charles/pypath/peewee.pyc in execute_sql(self, sql, params, require_commit)
   2711             cursor = self.get_cursor()
   2712             try:
-> 2713                 cursor.execute(sql, params or ())
   2714             except Exception as exc:
   2715                 if self.get_autocommit() and self.autorollback:

OperationalError: no such table: user

In [5]: db.drop_tables([User], safe=True)

In [6]: db.drop_tables([User], safe=True)
```

I believe you can just ignore the warnings.

Just ugraded to version 2.3.0 and I was trying the `drop_tables` function. If a table doesn't exist then I get a warning like the following (even specifying the `safe=True` parameter)

```
/mypathto/env/site-packages/peewee.py:2702: Warning: Unknown table 'variables' cursor.execute(sql, params or ())
/mypathto/env/site-packages/peewee.py:2702: Warning: Unknown table 'companies' cursor.execute(sql, params or ())
/mypathto/env/site-packages/peewee.py:2702: Warning: Unknown table 'users' cursor.execute(sql, params or ())
```

Is there a way to turn off the warnings when trying to drop a table that doesn't exist and when specifying the `safe=True` parameter?
