Adding possibility present prompt with custom UI + exposed ratingConditionsHaveBeenMet() func to public interface

[ibru]

I've introduced new property ratingPrompt and a protocol ArmchairRatingPrompt which defines how rating prompt is shown. And provided 2 default implementation for UIlAlertView and UIAlertController.

Default implementation works exactly same as before (tested on Example app). You can specify your own prompt UI like this:

let feedbackPrompt = MyFeedbackRatingPrompt(delegate: Armchair.defaultRatingPromptDelegate)
Armchair.ratingPrompt(feedbackPrompt)

-----

class MyFeedbackRatingPrompt : ArmchairRatingPrompt {
    // controls custom rating prompt UI
}

I'm not familiar with OS X development so I made the code iOS only. Didn't want to mess up OS X code.

I can also update iOS Example app to include example of using custom rating UI, if you want.

I also needed to know if rating conditions have been met, so I introduced new static func which exposes private ratingConditionsHaveBeenMet() to public.

[coneybeare]

I can't add this without OS X, but I don't really have time to look into it at the moment. You may find it is not too hard to add it to the OS X code as the language is the same and API's are similar. Consult the example project.

[phimage]

I make on @ibru repository a PR for OSX ibru#1