Patch: correct LaTeX error in Linear Regression homework

The linear regression problem set uses an undefined macro, \sd.  This patch
removes that macro and just uses 'sd'.  It's not a fantastic workaround,
but the LaTeX code in that part of the document needs to be heavily
refactored anyway.
Merge branch 'patch_linearregression_problems' into dev

The linear regression problem set uses an undefined macro, \sd.  This patch
removes that macro and just uses 'sd'.  It's not a fantastic workaround,
but the LaTeX code in that part of the document needs to be heavily
refactored anyway.
Rename file for probability introduction
Add references to bibliography and correct date for Casella and Berger
Add probability references to introductory section
Remove section numbering from ADDENDUM of License

This change makes the license stylistically in line with the FSF's
original copy.
Move licensing documentation to a standalone file

To save on paper and space, I've moved the GNU FDL to a separate pdf and
taken it out of the individual documents.  This seems consistent with
Section 6 of the license (Collections of Documents), since this entire
repository is best viewed as a single collection.  I've commented out the
LaTeX code to include the license in the individual pdfs, so if anyone
wants to distribute any of them on their own it should be very easy to
reintroduce the text and comply with the FDL.
Move introductory material to its own booklet

It seems useful to have a list of all of the potential supplementary
textbooks in one spot, so I moved the overall motivation and introduction
to the textbook into its own file.