Add coq-HoTT as a dependency for the .vo files

Some basic testing suggests that this will cause `make` to auto-re-coq
the .v files whenever you `git submodule update`, i.e., when it's
possible to tell that the version of Coq we're using has changed.
Merge pull request #464 from JasonGross/coq-HoTT-depend

Add coq-HoTT as a dependency for the .vo files
Example 3.6.2 added
Exercises 3.2 and 3.3 added
backrefs  Example 3.6.2, Exc. 3.1-3
names updated to Book_X_Y_zzzz
Trunc in Basics