How to create invisible lamps in blender cycles

When you use Blender Internal, you usually use lamps to light up your scene. However, in Cycles, this is not the case. You have to use a plane, and add an emission shader on it. However, this can look ugly in reflections, such as mirrors.

Take a look at these two images:

 You can see that there is an ugly lamp behind, from the mirror's reflection.

Here, you can see that there is no lamp, the sky is black, but there is still lighting.

Here's the node setup for the invisible lamp.

There you have it!