Fixed undefined mixin errors within layouts
Fixed more undef
And fixed one last mixin error
Merge branch 'master' of https://github.com/duchenerc/stencils

Conflicts:
	core/_grid.scss
	layouts/display/_block.scss
	layouts/display/_flipbook.scss
	layouts/footer/_river.scss
	layouts/footer/_tricolumn.scss
	layouts/headernav/_singular.scss
	layouts/main/_blog.scss
	layouts/main/_listing.scss