Add an onImagesReplaced event

- event gets called after Imager.js replaces the src of images
- add test for onImagesReplaced callback
documentation for the onImagesReplaced callback
Improved callback signature.

- passing `images` as an argument of the callback
- creating the spy before Imager to ensure it will never run before Imager setup
- testing callback arguments passing
Refactoring fixtures cleanup.