exception when cloning a FK
ran into this today...

``` python
class Post(pw.Model):
  title = pw.CharField()
  author = pw.ForeignKeyField(Person)

Post.author.alias('derek').clone()
```

throws:

```
Traceback (most recent call last):
  File "test.py", line 30, in <module>
    Post.author.alias('derek').clone()
  File "/home/derek/projects/coleifer-peewee/peewee.py", line 469, in clone
    inst = self.clone_base()
  File "/home/derek/projects/coleifer-peewee/peewee.py", line 1342, in clone_base
    related_name=self.related_name,
AttributeError: 'ForeignKeyField' object has no attribute 'related_name'
```

looks like `related_name` is only set when `add_to_class` is called.
