let the Meta class can extends from another class
I can see the reasoning behind this change, but I'm loth to introduce any changes that might break existing applications. I'm not sure off-hand, but given the way peewee munges `Meta` objects at present, there may be some edge-cases or situations where this breaks.

What do you think?
 I see, the `dir()` function may be unstable.
Can we use `inspect.getmembers()` instead? 
http://stackoverflow.com/a/8529470/2815178
I think it's more safety.

``` python
import inspect
for k, v in inspect.getmembers(meta):
    if not k.startswith('_'):
        meta_options[k] = v
```
 Or I can just use a decorator:

``` python
def with_database(cls):
    cls.database = db
    return cls


class BaseModel(Model):
    @with_database
    class Meta:
        pass

class ModelA(Model): 
    pass

class ModelB(Model):
    @with_database
    class Meta:
        db_table = "model_b"
```

Then, nothing need changes in Peewee.
It's just not so nature.
 You do know you can write this, right?

``` python
db = PostgresqlDatabase("mydb")

class BaseModel(Model):
    class Meta:
        database = db

class ModelA(BaseModel): 
    # no need to define Meta / database = db, it is inherited.
    pass

class ModelB(BaseModel):
    class Meta(BaseMeta):
        # database = db --> this is inherited via BaseModel
        db_table = "model_b"
```
 Oh, I don't know this before. Thank you!
Perhaps this pull request is not needed any more?

Example:

``` python
db = PostgresqlDatabase("mydb")

class BaseMeta:
    database = db

class BaseModel(Model):
    class Meta(BaseMeta):
        pass

class ModelA(Model): 
    pass

class ModelB(Model):
    class Meta(BaseMeta):
        db_table = "model_b"
```
