Texture Paint Layer Manager [REMOVED].
Just in case this stumps anyone, at the time of writing (Blender v2.76) the Texture Paint Layer Manager add-on seems to have been removed - as of 2.74, in fact (https://developer.blender.org/T43364).
As an alternative, check out the Painting Base Colors chapter of the 'Texturing and Shading a Sci-Fi Helmet' course, where Kent gives a good explanation of how to use texture layers in the updated texture paint system.
That should be enough to get you going will a similar workflow, though admittedly, I haven't yet worked out how to fine-tune the display of a layer using the new system (for example, changing brightness, contrast or saturation, etc). If anyone knows if that's possible, let me know! ^_^
