Part 1: Basic skin
This approach is based on http://www.blendswap.com/blends/view/67519 which is licensed CC-BY-SA. The original file uses vertex colors for things that can't be done procedurally. I replaced vertex colors with masks using the default UV-layout. This way the skin fits your exported character without additional work. I used a scale of 1m = 1BU. If you use a different scale you will have to change some settings, especially those which rely on generated texture coordinates.
Let's have a look inside.
Ok, it looks a bit complicated, but it isn't. Let's start with displace section. It's just a voronoi texture using generated coordinates. The color ramp maps the voronoi pattern to values between grey and black since we only want negative displacement. RGB Mix controls the amount of displacement.
The diffuse part is just multiplying colors generated by noise textures with different scalings.
1. mask spots
This node uses UV coordinates and an image texture to control freckles. Here's what it does.
2. group_skin_noise
Two noise textures and a voronoi texture provide the basic skin pattern. The result gets darkened depending on mask spots.
3. Skin_shader
This could use a bit of improvement. I really still have a lot to learn about SSS.
This is our result so far.