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')})
```
Sure,  I don't mind. The doc wasn't clear. So I changed it to one option. Your choice.