Another attempt at Single Not Nodes

I just updated the Subversion repository on SourceForge. I'm still writing unit tests to prove that it works but I've still got some way to go.

I should also mention that now that I've got the Test Node working I tried solving SEND + MORE = MONEY using pyRete but I think it ran out of memory. ;-) I need to look at the Token class again. I actually just made it a subclass of the built-in Python dictionary (dict) because it was the fewest lines of code that did the job. But apparently it's not good enough anymore, at least not if I want to try some more demanding tests.