Draft implementation of CTRL-C fix for Win32.

Fixes issue #100.

This is a preliminary pull request to generate discussion.  Proper integration in documentation to be refined, testing on Linux/etc. TBD.
The problem is I can't import relevant stuff from `ctypes` unless `os.name == 'nt'`.  Importing `zmq.utils.win32` directly in application code would push the platform test into the application.  I want to make sure that people who use the context manager can use it on all platforms without going through any special hoops.  Any suggestions for achieving this without changing `zmq/utils/__init__.py`?