group_by() broken for queries without aliases
This code:

``` python
from peewee import *

class Message(Model):
    author = CharField()
    body = TextField()

print Message.select().group_by('author').sql()
```

Throws this exception:

```
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    print Message.select().group_by('author').sql()
  File "peewee.py", line 1216, in sql
    group_by = ', '.join(group_by)
TypeError: sequence item 0: expected string, tuple found
```

This part of SelectQuery.sql() seems responsible:

``` python
        if self.use_aliases():
            (...)
        else:
            group_by = [c[1] for c in self._group_by]
```

c[1] is a sequence of field names, not a single field name, so str.join() fails. The code used when use_aliases() returns True seems to work better.
