Use relative paths for images

[Anahkiasen]

The way the images are included at the moment with absolute URLs:

background: url("/images/sy-loader.gif") 50% 50% no-repeat;

Make it very hard to pack Slippry with something like Webpack, couldn't relative URLs be used there, to more easily let build systems know where the images are?

[sftsk]

If you use the sass version you can easily overwrite this:

$spinner_url: '/images/sy-loader.gif' !default;
$arrows_url: '/images/arrows.svg' !default;

There has been talk about this in #62 already, perhaps relative would be easier for some?

[mgsmus]

How about move images folder to dist and use "images/" instead of "/images/" ? Final folder structure could be like this:

slippry.css
slippry.min.js
images/