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.
Explicite prompt won't be showed, if user has declined to rate the app.
Merge pull request #106 from amjaliks/features/showprompt

Added call to show the prompt (the rating alert) explicitly
Merge branch 'master' of git://github.com/arashpayan/appirater