Allows to change the current Lang (Java API)

[julienrf]

#174

Usage (in a Java controller):

public static Result setLang(String code) {
    changeLang(code);
    return ok(…);
}

• Add a Http.Context#changeLang(String code) method ;
• Add a Controller#changeLang(String code) shorthand ;
• Let the developer use a custom cookie name to store the user’s lang, through the application.lang.cookie configuration key (defaults to PLAY_LANG).