Bug 863066: Changes for Firefox OS newsletter launch

- Check both Master_Subscribers and OPT_IN before concluding a user
  does not exist in Exact Target
- Make more explicit behavior on subscribing in various
  cases
- Add support for translated welcome messages
- More tests
- Other refactoring and cleanup
Merge pull request #43 from dpoirier/firefox_os_changes
Bug 874616: Validate newsletter language

In the basket API, validate that incoming languages are either
blank or one of the languages supported by some newsletter.
Merge pull request #46 from dpoirier/bug-874616-validate-newsletter-language
Improve newsletter admin interface.
Merge pull request #47 from pmclanahan/improve-newsletters-admin
Merge branch 'master' into django-1.5

Conflicts:
	news/tasks.py
	news/views.py
	requirements/dev.txt
	settings.py
	settings_ex.py