Preview mechanism for emails (second round)

Hi again,

A better solution without the PreviewSite and a **preview_map** directly in the factory.

I have also added some docstrings.

Tell me your thoughts and if it's ok I will update the sphinx documentation with a new chapter on this.

This PR requires the previous [one](https://github.com/novagile/django-mail-factory/pull/12).

Bisous.