Aggregate(...) and fn.Max for DateTimeField should return datetime objects
Unfortunately, I'm not sure there's a great solution for this without resorting to hacks.  It would be possible to special-case flat aggregation / annotations by introspecting the field type and then passing the value from the cursor through the field's `python_value` method, but I'm not sure how clean the implementation would be and it might be fragile.  The other thing I want to avoid is a performance penalty.

For now I am going to leave this open, but I'm not sure that I will be able to resolve it.

Note:  This problem only appears in Sqlite and not mysql / postgres.
 Partially fixed in d23619abeb0f78d46284658d012970a30c7e3186 .  `annotate` works now, just `aggregate` / `scalar` fail.
 That's great, thank you. Do you plan to do a dot release soon?
 Within the next day or two.

I'm using Peewee 2.1.2, Python 2.6.1 with bundled sqlite3, see versions below: 

```
>>> sqlite3.sqlite_version_info
(3, 6, 12)
>>> sqlite3.version_info
(2, 4, 1)
```

Test case to reproduce the problem:

``` python

from datetime import datetime
from peewee import *

the_db = SqliteDatabase(':memory:') 

class MyModel(Model):
    dt = DateTimeField(null=True)

    class Meta:
        database = the_db

if __name__ == '__main__':

    the_db.connect()
    MyModel.create_table()

    # Fill    
    now = datetime.now()    
    for _ in range(50):
        m = MyModel()
        m.dt = now    
        m.save()

    # This works
    q = MyModel.select()            
    for i in q:
        assert type(i.dt) is datetime

    # These both fails    
    r = MyModel.select().aggregate(fn.Max(MyModel.dt))
    print type(r)
    assert type(r) is datetime

    q = MyModel.select(fn.Max(MyModel.dt).alias('max_dt'))    
    for i in q:
        print type(i.max_dt)
        assert type(i.max_dt) is datetime

```

Output:

```
Underworld:$ python testcase.py 
<type 'unicode'>
Traceback (most recent call last):
  File "testcase.py", line 32, in <module>
    assert type(r) is datetime
AssertionError
```

Thank you.
