more control to show prompt

- user facing:
  - added: +forceShowPrompt: (like showPrompt but no checks, with
    option to display rate later button).
  - added: +tryToShowPrompt (replaces showPrompt, more explanatory)
  - deprecated: +showPrompt
- inner workings:
  - added: -showRatingAlert: (option to display rate later button)
  - modified: -showRatingAlert (calls showRatingAlert:true)
  - added: showPromptWithChecks:displayRateLaterButton:
    (used by forceShowPrompt: and tryToShowPrompt)
no need to grab singleton within instance method
Merge pull request #157 from phest/forceshowprompt

More control to show prompt