Fix truncate_date for PostgreSQL
Cool, thanks!

The following script doesn't work:

``` python
#!/usr/bin/env python
import peewee, datetime

database = peewee.PostgresqlDatabase('mmwatch', host='localhost')

class Number(peewee.Model):
    date = peewee.DateTimeField(default=datetime.datetime.now)

    class Meta:
        database = database

database.connect()
database.create_tables([Number], safe=True)
number = Number()
number.save()
q = Number.select(database.truncate_date('day', Number.date))
for row in q:
    print row.date
```

It fails with the following error:

```
Traceback (most recent call last):
  File "./date_trunc_pgsql_fail.py", line 19, in <module>
    for row in q:
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 2967, in __iter__
    return iter(self.execute())
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 2960, in execute
    self._qr = ResultWrapper(model_class, self._execute(), query_meta)
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 2656, in _execute
    return self.database.execute_sql(sql, params, self.require_commit)
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 3492, in execute_sql
    self.commit()
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 3316, in __exit__
    reraise(new_type, new_type(*exc_args), traceback)
  File "/Users/ilyazverev/Progr/git/mmwatch/mmwatch/venv/lib/python2.7/site-packages/peewee.py", line 3485, in execute_sql
    cursor.execute(sql, params or ())
peewee.ProgrammingError: column "day" does not exist
LINE 1: SELECT DATE_TRUNC(day, "t1"."date") FROM "number" AS t1
                          ^
```

The reason is that `'day'` is passed to the query builder as-is, not escaped as a string. This pull request fixes that.
