Added call to show the prompt (the rating alert) explicitly.

In some cases you may want to show the prompt for rating explicitly regardless Appirater settings.
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;
Explicite prompt won't be showed, if user has declined to rate the app.
Merge branch 'features/customtitles'