Add ALPS SKRK series button footprint (finish #456)
This finishes #456
Product page: http://www.alps.com/prod/info/E/HTML/Tact/SurfaceMount/SKRK/SKRKAHE020.html
This finishes #456
Product page: http://www.alps.com/prod/info/E/HTML/Tact/SurfaceMount/SKRK/SKRKAHE020.html