Extra manufacturers type class

This contrib should eventually add another "manufacturers" style filter to your site - I am using it on my own store to add more functionality - we sell jewellery, and I already use "manufacturers" to distinguish between different precious metals (gold, silver, platinum etc.), and want to have another set to sort by designer of each type of jewellery, as many ranges use different metals within the range, and many metals have more than one designer - I think this could be used in many situations such as a bookstore could be sorted by author and publisher, or a computer store could have manufacturer and operating system, a clothes store could have materials and label etc.

This is being used outside of the category tree, as both metals and designers combine within each category within my site and I am trying to avoid thousands of sub categories, as well as the fact that customers shop in many different ways - some by metal (manufacturer), some by designer, and some by type (i.e. Celtic Pendants) which is covered in my catagories array.

Not being a great php coder, I have only implemented this to the admin side of the store so far, and am stumped on the catalog side - I have tried for many an hour, but am hoping that by posting this as a partial contrib I can enlist the help of other developers who may also have a use for this kind of filtering! If and when I manage to get it working, I will update with the full package, but I am sure the code can be cleaned up a bit and that the catalog functionality should be completed. As with all contribs, please ensure you back up completely before doing any changes.