[Build] Fix build on Windows

* In the prepublish step, run bower and gulp via node, instead of
  relying on shebang interpretation. (Forward-slash path separators
  appear to get normalized by node itself before executing the scripts.)
* In the gulp build, replace hard-coded *nix-style separators with
  path.sep; this allows stylesheets to be output to expected locations
  when building on Windows.

Addresses #827.

gulpfile.js

@@ -91,7 +91,8 @@ gulp.task('stylesheets', function () {
         .pipe(sourcemaps.init())
         .pipe(sass(options.sass).on('error', sass.logError))
         .pipe(rename(function (file) {
-            file.dirname = file.dirname.replace('/sass', '/css');
+            file.dirname =
+                file.dirname.replace(path.sep + 'sass', path.sep + 'css');
             return file;
         }))
         .pipe(sourcemaps.write('.'))