Merge pull request #67 from jkuczm/fix_message_preview_template

Added send_html model to toggle usage of HTML templates.

Fixes #64, #67 and paves the way for #66.
Updated Transifex host.
Slight spelling error.
Updated translations and messages.
Added Russion translations.

Thanks to Eugene Akentyev.
Change message templates select order

Message template overrides for specific newsletters were never used,
because "global" message templates were always loaded first.
Move common email tests to a mixin

* Create AllEmailsTestsMixin performing tests for all message types.
* Adapt HtmlEmailsTestCase and TextOnlyEmailsTestCase to inheritance
from AllEmailsTestsMixin.
Customize Newsletter creation in MailingTestCase

* Add get_newsletter_kwargs method to MailingTestCase.
* Adapt MailingTestCase.setUp to use
MailingTestCase.get_newsletter_kwargs.
* Adapt HtmlEmailsTestCase and TextOnlyEmailsTestCase to use
get_newsletter_kwargs.
Remove MailingTestCase.fixtures

It contained only 'default_templates', fixture that was removed in
30d7aaa.
Add test template overrides
Add tests for templates overrides

* Add tests.utils.template_exists function.
* Add assertEmailAlternativeBodyContains assertion.
* Add TemplateOverridesTestCase testing template overrides for subject,
text and HTML body for all message types. Skipped if templates overrides
don't exist.