Implement support for user-set defaults.

For everyone who has ever had to work with models like this:

```
class Foo(models.Model)
    field1 = MoneyField(max_digits=12, decimal_places=2, default_currency=settings.DEFAULT_CURRENCY)
    field2 = MoneyField(max_digits=12, decimal_places=2, default_currency=settings.DEFAULT_CURRENCY)
    ...
    fieldN = MoneyField(max_digits=12, decimal_places=2, default_currency=settings.DEFAULT_CURRENCY)
```

I submit a humble pull request which allows you to set defaults for `max_digits`, `decimal_places`, and `default_currency` using the variables `MONEYFIELD_MAX_DIGITS`, `MONEYFIELD_DECIMAL_PLACES` and `MONEYFIELD_DEFAULT_CURRENCY` in `settings.py`.

If you see anything about my code that's worth pointing out - feel free to let me know. Cheers (: