Model: make $incrementing protected

What is the reason behind `$incrementing`, `$timestamps` and `$exists` being public in the model?

See https://github.com/laravel/laravel/issues/1453 for reference.

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Note that I changed the `getIncremting()` method to `isIncrementing()` because that's more consistent with other methods.

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Should I add helpers for `$timestamps` and `$exists`, too? For the timestamps, the getter already exists (`usesTimestamps()`).