added exercise to chapter 3
added reference to old exercise in chapter 3
Tweaks to allow latex (and not only pdflatex) to compile the book

This is required for getting htlatex to work, for an eventual
html/non-MathML-epub target.  The main changes are:
(1) we use \DeclareGraphicsExtension and never pass .png explicitly
now
(2) we patch \includegraphics when not in pdf mode to read bounding
boxes using bmpsize-base, to silence "can't determine size" errors
moved new exercise to the end
Add another \texorpdfstring
Merge pull request #661 from JasonGross/fix-dvi-mode

Tweaks to allow latex (and not only pdflatex) to compile the book
Merge pull request #660 from nicolaikraus/master

added exercise to chapter 3
fix typo in theorem 7.7.4
errata for cd8a902
Merge pull request #663 from JasonGross/texorpdfstring

Add another \texorpdfstring
Merge pull request #665 from mikeshulman/theorem7.7.4

fix typo in theorem 7.7.4
Mark Errata (auto)