Included instructions for running Python 2.7 and 3.3, and setting up 2.7 and 3.3 versions of IPython Notebook.
PHS story.
Merge branch 'master' of https://github.com/ehmatthes/intro_programming
Removed redundant	line of code and added missing space
Started numerical lists, range function.
Merge pull request #16 from lukecyca/typos

Removed redundant line of code and added missing space
Merge branch 'master' of https://github.com/ehmatthes/intro_programming
Clarified negative list indices, range() function.
Added min(), max(), sum() functions.
Added link to min_max_sum section.
Numerical list exercises.
List comprehensions.
Fixed links.
Formatting headers and links.
Fixed link.
Clarified setup instructions.
Started strings as lists.
Merge branch 'master' of https://github.com/ehmatthes/intro_programming
Reorganizing strings section.
Added 'Working Backwards' exercise.