Add send_html field to Newsletter model

* Add send_html field to Newsletter model
* Add South migration
* Change Newsletter.get_templates to use send_html field
Fix message preview template

HTML preview of message was not shown due to condition depending on
non-existing attribute message.newsletter.message_template.html.
Change it to message.newsletter.send_html.
Add tests for HTML and text only emails

* Add to MailTestCase assertions for testing email alternative content
types.
* Add SendEmailsTestCase a base TestCase class for testing sent e-mails.
* Add tests of email alternative content types for subscription,
unsubscription and update confirmation emails and for message email, for
newsletter with send_html=True/False.