Use strict inside functions

[rjwaring]

Keep 'use strict' inside functions, otherwise it may affect other scripts on the page. See https://github.com/boxuk/angular-symfony-translation/blob/master/src/app.js#L1

[TomSeldonBoxUK]

Will investigate, but first thought is that shouldn't be an issue in this case as when compiled everything is placed in an anonymous function to avoid polluting the global scope.