Support for BigIntegerField / bigint.

[elver]

I had a problem where I was using a BigIntegerField as the 'id' field for certain tables and that column was not being created when doing a syncdb. So I did a tiny patch to fix that. Works now.

Here's a minimal example of how it can break without the patch:

class ObfuscatedPKModel(models.Model):
    class Meta:
        abstract = True
    id = models.BigIntegerField(primary_key = True, db_index = True)