Added gridExtra package to required library list

grid.arrange used in the example comes from the gridExtra package
fix typo in R^2 math

Should be sum of squared X deviations, not just sum of X deviations
Fix a typo "corrrelation"
Merge pull request #17 from mmulchandani/master

Modified hw2-4 and hw4-3
Merge pull request #16 from gostevehoward/master

Fix a couple of typos in regression models residual variation slides
Merge pull request #14 from tksf/patch-2

Added gridExtra package to required library list
Tried fixing the display problems