Join doesn't work when a composite key is used
I tried a raw SQL query and it works fine:

``` SQL
SELECT * FROM image JOIN treatment WHERE image.date == treatment.date
```

The database backend is SQLite.
 Have you tried explicitly setting the join condition?

``` python
predicate = (Image.date == Treatment.date) & (Image.time == Treatment.time)
 query = Image.select().join(
    Treatment,
    on=predicate)
```
 That works by setting the join condition. I thought I could set it using where, but that's not the right place.

Thanks!

Hello,

I have a table that has a primary key set as a Composite Key and I am unable to complete a join.

Here is the traceback:

``` Python
Traceback (most recent call last):
  File "localsync.py", line 79, in get_dates
    query = Image.select().join(Treatment).where(Image.date == Treatment.date)
  File "C:\Python27\lib\site-packages\peewee.py", line 257, in inner
    func(clone, *args, **kwargs)
  File "C:\Python27\lib\site-packages\peewee.py", line 2077, in join
    raise ValueError('A join condition must be specified.')
ValueError: A join condition must be specified.
```

and here are the models:

``` Python
class Image(Model):

    pid = CharField()
    date = DateField()
    time = TimeField()
    panel = CharField()

    class Meta:
        database = db
        primary_key = CompositeKey('date', 'time')

class Treatment(Model):

    id = PrimaryKeyField()
    pid = CharField()
    date = DateField()
    time = TimeField()
    field = CharField()

    class Meta:
        database = db
```
