meta_options are overwritten by clauses in child-classes
Thanks for reporting this -- yes, peewee replaces the `order_by` string with a clause, but this is acting up in the chlid-classes. I will get a fix out soon.

Hi Charles,

I think this example should be valid but is having difficulties:

``` python
from peewee import *

db = SqliteDatabase(':memory:')
db.connect()

class BaseModel(Model):
    created = DateTimeField()

    class Meta:
        database = db
        order_by = ('-created',)

class Extension(BaseModel):
    some_field = CharField()
```

Gives the following error:

```
alexs-air:Code alexlatchford$ python test.py 
Traceback (most recent call last):
  File "test.py", line 13, in <module>
    class Extension(BaseModel):
  File "/Library/Python/2.7/site-packages/peewee.py", line 2990, in __new__
    cls._meta.prepared()
  File "/Library/Python/2.7/site-packages/peewee.py", line 2861, in prepared
    field = self.fields[clause.lstrip('-')]
AttributeError: 'DateTimeField' object has no attribute 'lstrip'
```

Sorry I haven't had time to investigate it and find a patch, I'll see what I can do if this really annoys me. I'm just putting the order_by declarations on the sub-classes instead of the parent :)

Thanks,
Alex
