Sub Category Search Suport

This is a pull request to replace #160 and #172

Originally discussed in the google group:

https://groups.google.com/forum/?hl=en#!topic/python-soco/tFnNlGTp9TU

I know there were comments about looking at a different approach, and I'm not saying I disagree with them. I am publishing this for 3 reasons:
1) The #160 request has other python 2.6 changes in it (#172 had other changes that should not have been there)
2) I happened to need it for my system when upgrading from 0.8 (So saw no harm if updating the pull request)
3) This functionality was requested again: #182

In an ideal world it would be good to get it in for 0.9