This module is a "throw nothing" version of zmq.hpp.

Each object type must be initialized with a call to the appropriate
init() member function after creation and before use, infra vide.

Other changes:

 operator void*() has been added to context_t to allow interoperation
 with standard zmq_ calls

Motivation: 

zmq:: objects are very useful, however in legacy systems with mixed 
C/C++ code bases, try-catch is often not fully implemented throughout 
the system.  

This module provides the advantages of class-based access
to ØMQ without creating the need to pepper try-catch statements 
throughout the codebase.

Updates:

- Per Martin S. added a separate "in use" flag