Clang doesn't like empty initializers for maps and sets :(
Ignore zmq header
Completely remove references to empty initialisers

This should definitely solve the issue we're having with the clang
compiler complaining about explicit constructors being set and not
being called properly when using empty initialisers for sets and
maps.
Merge pull request #73 from massix/bugfix/clang

Clang doesn't like empty initializers for maps and sets :(