Invoke lab with same node args used to run gulp.

Currently `gulp-lab` assumes that the `node` executable is called "node" and that it is invoked without additional options. This can cause problems if a system uses `nodejs` as the node executable or if `gulp-lab` is being used to test a project leveraging the harmony extensions.

This change updates `gulp-lab` to pull the node process name and arguments from the `gulp` process.