Replace execfile with something that works under both Python 2 and 3

Execfile doesn't exist in Python 3, this commit replaces the use of
execfile with an equivalent construct known to work under both versions
of Python.

The change is actually the result of running 2to3 over setup.py, I just
added a note to explain its presence.
Fix name of reply header

After looking at the code (while working on a haskell implementation), I noticed that it uses 'response_to' rather than 'reply_to' as the response message header.
Merge pull request #101 from cpennington/master

Fix name of reply header
Merge pull request #99 from lvillani/python-3

Replace execfile with something that works under both Python 2 and 3
Added the latest versions of pyzmq to the build process.
Merge pull request #103 from thedrow/patch-1

Added the latest versions of pyzmq to the build process.
wip