Merge pull request #30 from hintjens/master

Fixed some errors in the text
Merge pull request #31 from hintjens/master

Identity on DEALER-to-ROUTER connection is 16 octet binary
Merge pull request #32 from hintjens/master

Added some further reading
Merge pull request #33 from hintjens/master

Broadcast port zero to remove self from network
Merge pull request #34 from hintjens/master

Fixed error in header layout
Merge pull request #35 from hintjens/master

Added ERROR command
Added print methods as optional standard
Added support for MPL 2
Fixed spec states