Use execFile instead of exec when calling xdg-open script

[BHSPitMonkey]

On Linux, open.js invokes vendor/xdg-open using exec with a command string.

Because xdg-open is a local script included with the module, this causes problems in Electron applications that are packed into an ASAR archive. (ASAR tries to resolve internal paths to files in the application, but doesn't know how to handle an arbitrary command string like the ones used by this module.) This specific pitfall is documented here: http://electron.atom.io/docs/tutorial/application-packaging/#executing-binaries-inside-asar-archive

I understand this isn't the fault of this module, but it nonetheless be very helpful to switch from exec to execFile when calling the local xdg-open script if possible.