Problem: getting a serialized integer from a zmsg is painful

Solution: add a simple decoder (zmsg_popint).
Problem: main README is badly formatted

There are numerous small issues in the source code (like blank line
at start of source) and the mkman script that cause the README.md
to be badly formatted.

Solution: fix these.
Problem: README TOC is mostly empty

Due to gitdown design the class sections don't appear in the TOC.

Solution: fix gitdown parsing.
Problem: TOC is hard to read

Solution: collapse only level 4 items
Merge pull request #733 from hintjens/master

Documentation fixes