Enhance Email notification on API docs changes

1. Trigger ACTIVEDOCS changes of add, update (replace existing one) and delete.
2. Any changes related to ACTIVEDOCS will send notifications.
3. User can have tick boxes to subscribe those events.
4. service provider and API consumers have different email content

Please see the code below:
We can add two more methods,  update_notification (replace existing one), delete notification
https://github.com/3scale/porta/blob/3017562449d3275840cae6afe338a01c1dcca899/app/models/api_docs/mailer.rb#L18