2018rev0

[#22] Strip down long- and remove unnecessary- examples
[#22] Explain multiple returns as a special case of multiple assignment
[#22] Move "importing modules" after "name assignment and namespace"
[#18] Fix binder files
[#13, #15 ] Add spell- and link- checking
[#10, #17, #20, #25] Minor formatting and content changes

2017rev1

[#5] Added nbtutor visualizations
[#6] Added nbtutor installation instructions
[#7] Updated the introduction lecture for the tutors
[#6, #8] Minor updates and typo fixes

2017rev0

install also for python3