Made mail view XSS-save using the safe-html transform for the mail-body display.
Allow <style> tags inside the mail body.
Parse <style> tags inside the mail using the `premailer` transform.

To avoid that styles defined in a `<style>` inside a mail, are executed for the
 complete page we parse them and add it to the tags itself, with the help of the
[premailer package](https://pypi.python.org/pypi/premailer).
Merge pull request #25 from 4teamwork/pg_html_safe_mailbody

Made mail view XSS-save using the safe-html transform for the mail-body display.