A mixin class to improve watchers declaration

Usage:

```python
from observer.mixins import ObserverMixin, CallMethod, CallFunction

class Book(ObserverMixin, models.Model):
    name = models.CharField(max_length=255)
    description = models.CharField(max_length=255)

    class Observers:
        name = CallFunction('name_changed')
        description = CallMethod('description_changed')

    def description_changed(self, sender, obj, attr):
        print 'description changed'

def name_changed(sender, obj, attr):
    print 'name changed'
```