python: special fallback includes on windows

On the windows the current fallbacks in setup.py do not make sense.
Include the dirs as if we run setup from the git checkout source, but
*only* if there is *one* such directory. Add a hint if there are two
or the msvc build was not build...
Add a python-msvc target and script and a INSTALL.WINDOWS instruction

The script can be used to build a python packages from cygwin
1) clone igraph
2) ./bootstrap.sh; ./configure;
3) make python-msvc

-> will build the igraph-*-msvc direcory, build it via msvc tools (must be
available!) and then the python bindings. The makefile uses the script
tools/build_python_windows.cmd

This currently only works with python 2.7 and after setting up msvc.
Merge pull request #532 from JanSchulz/python_windows_build

auxiliary build targets and scripts to help building igraph on Windows, thanks to Jan Schulz