Disable checking image dimensions

[akoufa]

Hello I opened PR: #479 to allow the users of the lib to disable the validation of the dimensions of the images to be added to the pass. I think the lib is currently too strict because testing the same assets with the apple tool I am able to create a pass. Nevertheless I extended the option object for users be able to pass the property disableImageCheck to disable explicitly the image check when needed. Can we proceed with merging the PR asap ? I need that functionality and it does not alter the default behaviour of the lib.