Added rating alert texts customization through delegate calls.

Default rating alert texts can be override by adding calls to delegate:

    -(NSString *)titleForAppiraterRatingAlert:(Appirater *)appirater;
    -(NSString *)messageForAppiraterRatingAlert:(Appirater *)appirater;
    -(NSString *)titleForRateButtonInAppiraterRatingAlert:(Appirater *)appirater;
    -(NSString *)titleForRemindButtonInAppiraterRatingAlert:(Appirater *)appirater;
    -(NSString *)titleForCancelButtonInAppiraterRatingAlert:(Appirater *)appirater;