Rounded Images

[migue1s]

I've added the kCRToastImageVerticalPaddingKey and kCRToastImageRoundedKey keys.

kCRToastImageVerticalPaddingKey adds a vertical padding to the imageview. kCRToastImageRoundedKey forces the image to be round.

These two keys allow creating notifications which have round icons even if the images themselves are squared (especially when used with the kCRToastImageContentModeKey key).

[lenkaiser]

Please merge this with the master branch.