Added a link to the python tutor for explaining the stack in a more dynamic fashion
removed nested function calling, IMHO this adds too much information to the basics
changes in text to separate a bit the idea of function composition from following the stack in a more complex example
removed in-place operators - never used in the lesson! is it really necessary?
Solve typos in functions and loop notebooks.
Merge pull request #60 from juliangarcia/gh-pages

Python tutor links + consider removing some material