Windows support

Pymatbridge can be successfully used on Windows.

messenger.c now can be compiled into a mex with visual studio
add windows binary MATLAB messenger (mexw64)
Add windows installation instructions to README.md
Update README.md

Explain how to edit environment variable on windows.
prevent windows from being spawned on windows

Command line options were checking for sys.platform being "Windows", not
"win32"
Merge pull request #56 from aebrahim/win32

Windows support