Initialize counters when app started for the very first time

My previous attempt not to zero couters on update doesn't take into account the case when app is launched for the very first time - counters aren't set in this case, see

https://github.com/nicklockwood/iRate/commit/beb6898b89b865e967a27bc66e1af5952deebd05#commitcomment-5917963

This should fix it (again, untested, so please check before I introduce more issues).

I'm afraid this is a big enough problem that a new release should be made.