Max-width 100% and extra param for onImageLoaded

It was non-obvious that .ReactCrop--image-copy also needed max-width set as seen in #21. Also by providing the image DOM element to onImageLoaded the user could use the image dimensions to decide on an optimum crop (e.g. depending on landscape vs portrait)

Firefox compatibility update

• Make crop darkened background work on Firefox (FF doesn't support clip-path: inset)
• Use object-assign package and declare it as a peerDependency