Constants utility

I got the idea of a Constants utility from another project we work on. I thought it was a pretty cool idea that would allow us to use model constants in a more concise, consistent, and reusable way - while still allowing us to use typical Django conventions like `get_FOO_display()`. It's hardly necessary but I'm interested to see what others think.

```
class Entry(models.Model):
    STATUSES = Constants(
        unverified=('0', 'Not Verified'),
        verified=('1', 'Verified'),
        approved=('2', 'Approved'),
        invoiced=('3', 'Invoiced'),
        uninvoiced=('4', 'Not Invoiced'),
    )
    status = models.CharField(max_length=1, choices=STATUSES.choices(), 
                              default=STATUSES.unverified)

>>> entry = Entry.objects.create()
>>> entry.status
'0'
>>> entry.get_status_display()
'Not Verified'
>>> entry in Entry.objects.filter(status=Entry.STATUSES.unverified)
True
```