Add optional sentinel_value

MySQL ignores null columns when it comes to resolving unique index constraints. This means that all of our unique constraints that involve nullable columns are problematic. This will allow the `deleted_at` value to be set globally or per model.

```ruby
#pick some epoch
acts_as_paranoid sentinel_value: DateTime.new(0)
```

or globally in a rails initializer

`Paranoia.default_sentinel_value = DateTime.new(0)`