Query Error using APSW 
0e055df5317e9966831b760a5a2aa8990f759207
 Thank you for the excellent bug report, this should now be fixed.
 You may need to update your apsw version, because in the latest versions it _does_ support `fetchone()`. I had to revert my commit that fixed the issue, due to MySQL python driver not implementing the iterator protocol on its cursor... Anyways, just try upgrading apsw and you should be set!

When using the apsw_ext module, I receive the following error when performing queries:

```
>>> x = SomeModel.select().where(SomeModel.col1 == "test").exists()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "peewee.py", line 2896, in exists
    return bool(clone.scalar())
  File "peewee.py", line 2653, in scalar
    row = self._execute().fetchone()
AttributeError: 'apsw.Cursor' object has no attribute 'fetchone'
```

I see references in older versions of the code implementing a 'fetchone()' that calls next(), because APSW isn't DB-API compliant. Somewhere this code appears to have been re-factored out. 

This is using the trivial example model from the Docs

```
db = APSWDatabase(':memory:')

class BaseModel(Model):
    class Meta:
        database = db

class SomeModel(BaseModel):
    col1 = CharField()
    col2 = DateTimeField()
```

python 2.7.6 On Ubuntu 14.04
Using peewee 2.8.0 (git rev 4c1db1399c0906ea3ac59448fa4a333e813098fa)
Using python-apsw 3.8.2-r1-1ubuntu1 
