SecureDrop i18n

![screen shot 2014-11-16 at 1 53 23 pm](https://cloud.githubusercontent.com/assets/800071/5063346/026a783e-6d98-11e4-9ebb-18e240008a15.png)

- Integrates with Flask-Babel to add i18n and l10n support
- Languages are selected by Accept-Languages header or manual override
  by query parameter
- Adds a language selection menu in page footer
- Tags strings in source and journalist interface for translation
- Integrates with Transifex to collaborate on translation (https://www.transifex.com/projects/p/securedrop-app-code/)
- Fully translates into Italian and Traditional Chinese
- Setup vagrant to compiles translation files while running provision
- Unit tests

Todo:
- build_deb_packages.sh does not compile translation files
- Relative time format is not localized
Rather than continue with the deprecated `%` format string syntax, can we switch this (and any other similar uses of `%`) over to `.format`? I've recently been trying to replace every instance of `%` with `.format` whenever I come across them, but clearly there are still some stragglers.