Auto-incrementing BigIntegerField
As a workaround I overrode the `create` method. Please let me know if there is a better way.

``` python
class Dataset(BaseModel):
    id = BigIntegerField(primary_key=True)

    @classmethod
    def create(cls, *args, **kwargs):
        next_id = cls.select(fn.Max(cls.id)).scalar() + 1
        return super().create(id=next_id, *args, **kwargs)
```
 Yes but it's not built-in at the moment. You can create a new field type and register it with the db, or create a subclass of the bigint field and override the `__ddl__()` method. [Here are docs on creating a custom field](http://docs.peewee-orm.com/en/latest/peewee/models.html#creating-a-custom-field).

Going the former route, you might:

``` python
class AutoBigIntegerField(BigIntegerField):
    db_field = 'auto_bigint'

db = MySQLDatabase('mydb', ...)
db.register_fields('auto_bigint', 'BIGINT AUTO_INCREMENT')
```

I _believe_ that should do what you want.
 Thank you, cool stuff!
`fields` must be a dict and `register_fields()` did not work (KeyError on `create_table()`, so I did this:

``` python
database = MySQLDatabase(
    user=...
    ...
    fields={'auto_bigint': 'BIGINT AUTO_INCREMENT'},
)
```
 Oh right, yep just woke up... Cool well if that's working then I'm going to close. Please let me know if you are still encountering issues.

Hello,
I'm working on an application with a mysql database. Several hundred loggers will each send a measurement value every second for three years - so I have to use a BigIntegerField.
`id = BigIntegerField(primary_key=True)` works, but doesn't auto-increment. Is there any way to do this in peewee?
Thanks!
