Logic notebook update (#932)

* Added KB_AgentProgram and subst

* Added doctests

* Updated README.md

* Fixed doctest

* Fixed doctest

* Fixed doctest

* Added definite_clauses_KB to logic.py

* Fixed a doctest, again

* Fixed another doctest

* Fixed another doctest

* Moved unnecessary doctests to unit tests

* Added unit test for ModelBasedReflexAgent

* Added unit test for ModelBasedReflexAgent

* Updated README.md

* Minor fix

* Fixed a doctest