Add \RestfulInterface::publicFieldsInfo() for declaring fields.

Use \RestfulInterface::getPublicFields() to get the processed fields (i.e. with default values initiazlied).
Merge pull request #135 from Gizra/public-fields

Add \RestfulInterface::publicFieldsInfo() for users to declare their fields.

Also rename 'process_callback' to 'process_callbacks' and allow it to pass the value to multiple callbacks.