Make UAAppReviewManager more inheritance friendly

[tonyxiao]

Use [self class] rather than UAAppReviewManager explicitly.

Also maybe separate the class convenience methods into a separate file for those who want it and expose the actual instance methods for subclasses to override.

[coneybeare]

Good idea. Would you be willing to do the alterations to the class methods and submit a PR?

[coneybeare]

For clarification, I don't want to expose the instance methods, but they can be added manually in a private header of the subclass for those who want to subclass it. I do like the idea of using [self class] instead of UAAppReviewManager though.