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, a DeprecationWarning is wrong here. The thing is, if you don't set it in the model field, you won't be able to use `ModelForm`. Because it needs to be defined for the FormField.
Users should get warned and told why ModelForms aren't working. But one could throw this warning later. I'll fix that.