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
Update README.md
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
adds to_rule_cpd method in TreeCPD class
Merge pull request #165 from ankurankan/dev

adds to_rule_cpd method in TreeCPD class
Update README.md
Update README.md
Merge branch 'dev' of https://github.com/pgmpy/pgmpy into dev
added some docstrings, merged get_values in Factor, changed marginalize for self-use temporary, PEP8, changed __str__ for Factor temporarily,  removed debug statements, removed read_simple_format, made _add_jt_edges public, changed tests for get_observation when observation was not set