[ADD] add thumbnail images for machine image slider. updated related mixin, and grunt file.
[NEW] a lazy loading workaround for bg images used in slick image slider.

Using '.hidden' css class to display none to stop browser from fetching the bg images, and removing the class on section load.
[NEW] add loading gif for use with images lazy loading.
[NEW] style for images lazy loading placeholder loading gif.
[NEW] added placeholder style class for images + data-src attar for image src url.
Added machine type class for root div.
[NEW] Added js function for lazy loading images, triggered onLeave fullpage call back function, by matching upcoming section.