"Offline" support for ForeignKeyField (related_name)
The implementation of reverse relations is a SelectQuery, which requires a database call.

I think it would be nice if one could do something like below. So stepping though the "children" before saving them.

``` python
import peewee as pw

class Parent(pw.Model):
    name = pw.TextField()

class Child(pw.Model):
    parent = pw.ForeignKeyField(Parent, related_name='children')
    name = pw.TextField()

p1 = Parent(name = 'p1')
c1 = Child(parent = p1, name = 'c1')
c2 = Child(parent = p1, name = 'c2')

for child in p1.children:
    print child.name
```
