Issues/2 `on` ignored by migrations

Fix for issue #2 

Will release as new version `0.2.0`.

Any migrations in your project created with earlier versions are broken if you rely on the `on` feature... you'll need to recreate or manually edit the existing migrations.

__For Django 1.7+ migrations:__
*****
Look for any `CreateModel`, `AlterModelTable`, `AddField`, `AlterField` operations (maybe others) ...anything where the definition of an `ExclusiveBooleanField` which should have an `on` kwarg has been serialized for the migration apparatus.

eg
```python
migrations.CreateModel(
    name='ShorthandModel',
    fields=[
        ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
        ('value', models.IntegerField(null=True)),
        ('the_one', exclusivebooleanfield.fields.ExclusiveBooleanField(default=False, on=(b'value',))),
        # add the `on` kwarg (IMPORTANT: use the tuple form) -------------------------^
    ],
    options={
    },
    bases=(models.Model,),
),
```

__For South migrations:__
*****
(tested on Django 1.3-1.6 using South 0.8.4)
Since the state of the ORM is frozen as a big dict in every single migration file you're going to have to change in every migration since the field/model was added to your project.

eg
```python
'testapp.shorthandmodel': {
    'Meta': {'object_name': 'ShorthandModel'},
    'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
    'the_one': ('exclusivebooleanfield.fields.ExclusiveBooleanField', [], {'default': 'False', 'on': "('value',)"}),
    # add the `on` kwarg (IMPORTANT: use the tuple form) ---------------------------------------^
    'value': ('django.db.models.fields.IntegerField', [], {'null': 'True'})
},
```