Merge pull request #79 from eggrobin/master

Catch up.
Merge pull request #80 from eggrobin/lecture-2013-04-18

Missing parentheses.
Merge pull request #81 from eggrobin/lecture-2013-04-16

Lecture 2013 04 16
Merge pull request #82 from eggrobin/lecture-2013-04-18

Lecture 2013 04 18
First draft of colophon, QR codes missing.
Colophon seems to be done, but somebody needs to review.
Merge the colophon