* added have_cake_and_eat_cake_too

* renamed effect_neg to effect_rem

* added have_cake_and_eat_cake_too to readme

* added details to Problems in planning.ipynb

Added more information for the problems Air Cargo, Spare Tire, Three Block Tower, Have Cake and Eat Cake Too.

* removed a test from planning.py

A test for three block tower problem is written here. I have removed it.

* Style fixes

* minor style fix

* fixed a typo

* minor fixes

some sentence issues

* minor changes

* minor fixes