Fix #48 change \[ and \] to $$

[rayfix]

Fix #48 change [ and ] to $$

I started off by running the following command:

  perl -0777 -pi -e 's/(\\\[|\\])/\$\$/g' *.Rmd

Then I backed out some places that were problematic.
I build the project and looked at a couple of the
Rmd files and the html output. Those looked good.

[rayfix]

I should mention that the stat 420 homework templates could also be fixed. (The script would probably do the job.)

[daviddalpiaz]

I'm actually going to close this for now. I'm going to be combing through the book this summer, and updating these manually will give me a reason to read it a little more thoroughly. (Also I'm nervous about making so many changes that I can't "see.)"