Don't depend on markup for lazy load the images

The lazy load strategy was depending on user's markup. The `src` tag could
not be present in the HTML, because Slick would understand it as loaded. Now,
it depends solely on `data-lazy` attribute: its presence will force the load,
and its absence will indicate it is already loaded.
Progressive lazy load now also do not depend on markup
Merge pull request #223 from bellthoven/lazy-load-markup-independant

Don't depend on markup for lazy load the images