integrate with strong parameters. Closes #41

Integrate with strong parameters. This allows to migrate a codebase partially
from `protected_attributes` to `strong_parameters`. Every model that does not
use a protection macro (`attr_accessible` or `attr_protected`), will be
protected by strong parameters. The behavior stays the same for models, which
use a protection macro.

To fully restore the old behavior set:

    config.action_controller.permit_all_parameters = true

Or add a callback to your controllers like this:

    before_action { params.permit! }

Closes #41.