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')})
```
haha, true, I refactored that for pep8 reasons.
That needs some fixing. Still doesn't explain why the tests are running fine locally but fail on travis.