Add util function to decode Facebook's `signed_request`
Add `get_best_locale` util function

This function gets the best locale possible from the `FACEBOOK_LOCALES` setting. It may either be the given locale, the main locale for the given locale's language or the default `en_US`.
Add view decorator to use Facebook locale

Gets user's Facebook locale from the `signed_request` parameter and compares it to the supported locales using `get_best_locale` function.
Add l10n to Download Tab

Uses `facebook_locale` decorator to get locale from Facebook. Wrap all copy strings in `_`and `trans` function calls.