late binding for encryption function

Some important things happens right after the content_object_init signal, e.g. the |filename| special function to resolve urls is applied to the content. So I want to encrypt the page as late as possible.

This patch removes the `encrypted_content` attribute for the instance and adds an `encrypt` function instead to the instance. The template is changed in order to use the `encrypt` function. So the encryption actually takes place when the content is rendered by the template.