Initial commit.

Create image-slider.js
Add object skeleton, with plan.
Set next, current and previous indices.
Add findImageContainers method to get elements from document.
Add setImages method to set images as backgrounds on elements.
Add next method, reference of last index in imageset.
Add previous method.
Add bindings, move container indexes to where containers are set.
Add self reference.
Call bind.
add chooseIndex method.
Refactoring, taking into account choose index.
Add sanity check to chooseImage
Add image preloading (experimental)
Add listener to only preload images when DOMContentLoaded