Model improvements

* Changed type of date fields to date time fields
* Fixed confirm_id length so the uuid fits in it
* Added __unicode__ methods so the models are display properly in the
admin tool
Registered models in the admin tool
Created module to send mails

* Based on markdown content
* Creates multipart mail (raw text AND html)
* Supports multiple receivers
* Displays unsubscribe link
* Optimises css for mail clients (inlines the css)