fixes #6

This should fix all open issues. There could be some unit tests added to the new interface. But hey ;)
You no longer need any Mixins, the Admin or even the FormField.

All one needs to do is:
```
MyModel(models.Model):
    width = MeasurementField(measurement=Distance, choices={('cm','cm')})
```
Although I have left things importable from `django_measurement.measure`, `Volume` and the other non-Unknown measures are all actually in the not-django-specific `measurement` library.  Could I convince you into leaving this pointing there?