add django support

[lamenezes]

it would be great if when working with django we could do something like:

from status_map.django import StatusField


class Task(Model):
    status = StatusField(status_map={'todo': ['doing'], 'doing': ['todo', 'done'], 'done': []})
    title = CharField(...)


>>> task = Task(title='implement django support on status-map')  # defaults to the first status passed (todo)
>>> task.status = 'done'
>>> task.save()
status_map.exceptions.TransitionNotFoundError: transition from todo to done not found

instead of something like:

from status_map import StatusMap

TASK_STATUS_MAP = StatusMap({'todo': ['doing'], 'doing': ['todo', 'done'], 'done': []})
class Task(Model):
    status = ChoiceField(choices=TASK_STATUS_MAP._map.keys(), default='todo')

    # not using validators because I'm pretty sure we can't acess the current status on the db
    def save(self, *args, **kwargs):
        task = Task.objects.get(pk=self.pk)
        if task.status != self.status:
            TASK_STATUS_MAP.validate_transition(task.status, self.status)
        return super().save(*args, **kwargs)