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')})
```
I'm sure you pursued all avenues for getting around this -- but I wonder why we'll need to have a measure pre-specified?

In at least a couple places that I'm using this library, I'm using it to store arbitrary measures and am rather hoping I won't have to refactor things; care to elaborate?