Changed vertical rhythm to use margin-bottom instead of margin-top

Signed-off-by: Dirk Jesse <office@highresolution.info>
Merge branch 'v4.1.0' of https://github.com/djesse/yaml4-sass into v4.1.0

Signed-off-by: Dirk Jesse <office@highresolution.info>

Conflicts:
	yaml/add-ons/rtl-support/core/base-rtl.min.css
	yaml/core/base.min.css
	yaml/core/iehacks.min.css
Fixed a bug in gradient mixing, fixed some wrong margins in the docs layout

Signed-off-by: Dirk Jesse <office@highresolution.info>