MonitorField value when the instance is created

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## Problem

MonitoField works without issues when the instance is updated. However, when an instance is created the value is not set.

e.g.

```
class MyModel(StatusModel):
STATUS = Choices('in_progress', 'success', 'error')

finished = MonitorField(
monitor='status',
when=['success', 'error',],
null=True,
blank=True,
default=None,
)
```

Then,
```
obj = MyModel.objects.create(status=MyModel.STATUS.in_progress)
print obj.finished # None ---> OK

obj.status = MyModel.STATUS.error
obj.save()
print obj.finished # `now()` ---> OK

obj2 = MyModel.objects.create(status=MyModel.STATUS.error)
print obj2.finished # None ---> Wrong
```

I tracked the error to the `pre_save` method, which is not considering the `add` argument.
Instead of checking `if previous != current:` it should be `if add or previous != current:`

The bug was introduced in [this commit](545bccf3ce63a29b0f29a1c620d88d72c3d873b8) when `StatusModifiedField` was migrated to `MonitorField`. Notice that the `_previous_status` method used to check the `add` argument to force the field to act.

Start at MonitorField's pre_save method and inspect how it handles the add argument. Compare it with the _previous_status behavior from the StatusModifiedField migration commit; done means creating an instance in a monitored status sets the value to now() while preserving the existing update behavior.

Written by the indexing model from the issue text.