fixes to src and binding.byp to allow windows installation

I was able to manually build node-mapserv on a Windows 7 64 bits machine with the following fixes.

The binding.gyp requires 2 variables to be manually set before configuring: 

    "variables": {
        'ms_buildkit%': 'Z:/PATH/TO/MAPSERVER_BUILDKIT',
        'ms_root%': 'Z:/PATH/TO/MAPSERVER'
    },