Cast to db_value for insert queries

I was getting problems with foreign key fields not being cast properly, looks like this is the solution. It's not well tested so you might want to have a play yourself before accepting.

```python
class MyModel(Model):
     uuid = UUIDField(primary_key=True)

class MyRelatedModel(Model):
     name = CharField()
     my_model = ForeignKeyField(MyModel)

my_model = MyModel.get(...)
MyRelatedModel.select(MyRelatedModel.my_model == my_model)
```
Should output something similar to:
```
HTTP Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/cherrypy/_cprequest.py", line 656, in respond
    response.body = self.handler()
  File "/Library/Python/2.7/site-packages/cherrypy/lib/encoding.py", line 188, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/cherrypy/_cpdispatch.py", line 34, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/Users/alexlatchford/Code/abbreviate/a8-product/a8_product/../a8_product/lib/auth_tools.py", line 11, in __ca
    return f(*args, **kwargs)
  File "/Users/alexlatchford/Code/abbreviate/a8-product/a8_product/../a8_product/routes/auth/signup.py", line 95, in POST
    NewsFilterKeyword.insert_many(news_filter_keywords).execute()
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2564, in execute
    return self.database.last_insert_id(self._execute(), self.model_class)
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2126, in _execute
    return self.database.execute_sql(sql, params, self.require_commit)
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 289, in execute_sql
    self.commit()
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2600, in __exit__
    reraise(new_type, new_type(*exc_value.args), traceback)
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 282, in execute_sql
    res = cursor.execute(sql, params or ())
ProgrammingError: can't adapt type 'UUID'
```

Thanks,
Alex