Feature for converting layers to greyscale

Hi,

Your plugin is a really nice tool for me and I use it a lot. Recently I found that I need some grayscale maps for a background layer. So I made this patch.
Basically, it provides a dialog showing layer properties where users can choose to convert a OpenLayer layer to show in grayscale. I am not so sure whether this is the best way of doing it. Please let me know whether you are interested. Thanks for your time!

Best,
spacelis