Update of pattern matcher context rules; porting of context rules to Python

@cosmoharrigan, I made some small changes to the pattern matcher rules and ported them to Python, renaming the old context_rules file. I implemented them in the most obvious way which is creating a separate rule for every case similarly to the pattern matcher. I think that this could be done more elegantly, but I wasn't aware of how exactly, particularly if a rule can be initialized with different values depending on the atomspace contents. I would welcome your insights and if these rules fit into the general format of the Python PLN rules.