Add public rateApp method to log rating events initiated from elsewhere

Hi, Andrew.

First, thanks for porting Appirater iPhone to Android.

I've ported the rateApp method (https://github.com/arashpayan/appirater/blob/371ffa39dd6a2a10309c11f30b3b2a01a43ca425/Appirater.h#L178) so that if there is a place elsewhere in the app for rating the app, it can go through the Appirater framework to log the fact that the app was rated.

Hope you find it useful.

Daniel