Solution for x-multiple-backgrounds image bugs

Solution for the issue: https://github.com/drublic/Sass-Mixins/issues/12

Also:
1) Add image of HTML5 logo for test case instead of abstract images.
2) In _multiple-backgrounds.scss remove extra -ms prefix in comments.
3) During grunt execution for mixins.scss: remove "// @include x-rem(font-size, 1.3);" and add fix https://github.com/drublic/Sass-Mixins/pull/14.