Allow changing accessible fields when creating Entity

Usage: ``$this->Entity->newEntity($this->request->data, ['accessibleFields' => ['*' => false, 'teste' => true]])``
Related to: #4215
Add test for accessibleFields option in Marshaller::one().

Add a test to test how the new option works in both the additive and
subtractive cases.
Add API docs for accessibleFields option.
Merge pull request #4565 from cakephp/marshaller-accessible-fields

Marshaller accessible fields