Add a windows-python target and script

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

-> 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
.gitignore cleanup and sorting