Some improvements to the command launching

[jmc-88]

I've replaced 'explorer.exe' with 'start' on Windows, which is more general and should work for all kind of URLs. I've also made 'xdg-open' the default handler for platforms other than Microsoft Windows or Mac OS X, since it seems like the other supported platforms should all have 'xdg-open' (http://nodejs.org/api/process.html#process_process_platform).