Added state for layman.

Adds the ability to ensure overlays are added or deleted via layman.

This is my first attempt at adding a state, so please let me know if I have any glaring mistakes.  I was able to use this to add and remove some layman overlays from a virtualenv install.  It should be rather helpful for configuring a state for a Gentoo machine, allowing salt to ensure the correct overlay is installed before trying to install a package that can only be found in the overlay.