fixed a small "typo"; reworded text for clarity
fixed typo
added challenge problems based on Python material
clarified challenge question

now has example of negative stride
string reverse challenge should be easier
edits to extra challenge: string reversal
added challenges to Lesson 1
added challenge problem: pattern-matching files
added challenges to the lesson on functions
reworded question about vertical lines in plots
reworded question about colon operator
more single colon rewording
reverting plot challenge question
Merge branch 'master' into adding_challenges_to_matlab_lessons

Conflicts:
	novice/matlab/03-func.md
Merge pull request #761 from jmxpearson/adding_challenges_to_matlab_lessons

Adding challenges to matlab lessons