Added serialisation for SortedSet and ListSet.

I added serialisation for immutable.SortedSet. I wanted to make it more generic but I couldn't figure out how to use CanBuildFrom and enforce the ordering. So any pointers on that would be welcome.

I threw in a fix for ListSet as well, since I was using it to test things.

Once this has been added, is it possible to propagate the fix to Scalding. I would like to use the CountMinSketch from Algebird which uses a SortedSet and currently that blows up.