Update setup.py

added Msgpack to required dependencies

My test on WinXP:
 1. Clean install of python-2.7.6.msi
 2. Install of setuptools-2.2.win32-py2.7.exe from http://www.lfd.uci.edu/~gohlke/pythonlibs/#setuptools
 3. Try to run setup.py install -> Run into http://bugs.python.org/issue9291 -> Used workaround http://bugs.python.org/issue9291#msg206938
 4. Try again to run setup.py install -> Run into issue http://stackoverflow.com/questions/20709641/cant-compile-msgpack-python-extension-under-windows -> Used workaround from http://www.borgelt.net/pycoco.html, i.e. typed
   SET VS90COMNTOOLS=%VS100COMNTOOLS%
  on cmd line to change compiler from VS2008 to VS2010.
 5. Try again to run setup.py install -> Compilation succeeds (with warnings) and setup is finished