Included free in the Blender file is my original PBR material node, and my Titan Space Hauler mesh.

--INSTALLING--

In order to install the addon, you go to File -> User Preferences -> Addons, and click "Install from File". Then, we'll navigate to the product directory, and inside we'll find the "easy_model_compositing_v1.0" zip. We don't need to unpack this file, just click the zip, and click "install". Then, scroll up and make sure to turn on the addon with the checkbox, and save your preferences.
**Special Note: In order to correctly capture shadows, the ground plane must have a material which has a Diffuse BSDF shader node in it; recommended is to use the include PBR material node.**

Sometimes a shadow pass will turn out noisy, with the ground plane visibly grey. This happens more often when using only environment lighting or when using very soft lamps. If this happens, just boost the contrast in your compositing software using RGB curves or a contrast filter. For example:

"Failed" shadow pass

Shadow pass cleaned in 5 seconds by boosting contrast

Due to the nature of the way that Cycles renders with raytraces and the Diffuse shader, currently this addon only really works for horizontal ground planes... more complex and vertical shadow catching geometry tends to self-shadow which muddies the final result.