NF: very basic notebook diff tool
Some small additions - up to Factors section
STY: a few typos
RF: edit to JB's means / sums explanation

Edit some typos
RF: moderate comments after Nathaniel's feedback
RF: some small edits to moderate conclusion

Add reference to Patsy.
RF: more edits from reviewing
NF: more typos
y_hats for the beta estimate explanation
Adding a gitignore for files created by nbconvert