minor change in doc and improve __str__ method
adds extra argument to cardinality so that it can now return cardinality of specific variable or a list of variables. Earlier returned a dict of cardinality of all variables
modified the __init__ method so that it sorts the variables before sorting into self.rules.
minor bug fixes in __init__ methods of TreeCPD and RuleCPD
adds to_tree_cpd method
Merge pull request #164 from ankurankan/dev

Adds to_tree_cpd method to RuleCPD class and some minor bug fixes