Centered gallery with margins

Signed-off-by: Zell Liew <zellwk@gmail.com>
centered gallery with padding

Signed-off-by: Zell Liew <zellwk@gmail.com>
complete gallery

Signed-off-by: Zell Liew <zellwk@gmail.com>
used nested mixin with gallery code

Signed-off-by: Zell Liew <zellwk@gmail.com>