Updated tutorial link under "Getting Started"

Old tutorial was deprecated, updated the link to point to the new location on "Getting Started" slide.
fix a couple of typos

Fixing the same typos I put up earlier in the .md file. Now that I've reached the right point in Reproducible Research lecture I realize I need to change them in the .Rmd file so they don't reappear
Merge pull request #13 from bschousek/patch-1

fix a couple of typos
Merge pull request #11 from tksf/master

Updated tutorial link under "Getting Started"
Added gridExtra package to required library list

grid.arrange used in the example comes from the gridExtra package
Added some requested zips
Merge branch 'master' of github.com:bcaffo/courses
fix typo in R^2 math

Should be sum of squared X deviations, not just sum of X deviations
Fix a typo "corrrelation"
Modified hw2-4 and hw4-3
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