Expose cropImage method

[dbergman]

I want to use my own UI design and use the cropping method provided in this source. By exposing the cropImage method I can use my own button and add cropImage as the target.

[ryang1428]

+1

[ruslanskorb]

I would like to see the following changes:

[1] Add the category RSKImageCropViewControllerProtected.
[2] Move the method cropImage to the category RSKImageCropViewControllerProtected.
[3] Update the comment for the method cropImage (Asynchronously crops the original image in accordance with the current settings).
[4] Extract and expose the method cancelCrop and the method reset: (=> RSKImageCropViewControllerProtected).
[5] Add comments for the method cancelCrop and the method reset:.

[dbergman]

@ruslanskorb I added RSKImageCropViewControllerProtected.