pwiz issue with composite primary keys
You should not specify `primary_key=True` if you are specifying a `CompositeKey`. Instead write:

``` python
class A(BaseModel):
    id1 = IntegerField()
    id2 = IntegerField()

    class Meta:
        db_table = 'a'
        primary_key = CompositeKey('id1', 'id2')
```
 Oh shit, should have read this more closely. You're right.

It generates something like

```
class A(BaseModel):
    id1 = IntegerField(primary_key=True)
    id2 = IntegerField(primary_key=True)

    class Meta:
        db_table = 'a'
        primary_key = CompositeKey('id1', 'id2')
```

which in turn leads to

```
ValueError: primary key is overdetermined.
```
