Get "raw" ForeignKeyField value without firing a query
`post._data['user']`
 Thank you Charles! :)

I haven't found a way to simply get related object id for ForeignKeyField without performing a query.

Is that currently possible? That would be convenient in cases when we need to know only related object id, i.e. when trying to read from cache etc.

Here's what I mean:

``` python
import peewee

db = peewee.SqliteDatabase('test.db')

class User(peewee.Model):
    username = peewee.CharField(max_length=100)
    class Meta:
        database = db
User.create_table(true)

class Post(peewee.Model):
    user = peewee.ForeignKeyField(User)
    class Meta:
        database = db
Post.create_table(true)

user = User.create(username="John")
Post.create(user=user)

post = Post.get(Post.id == 1)

# This will fire an extra query to get whole User object!
print(post.user.id)
```
