Added 'gulp' script, which does an 'npm install' when needed.

It then runs 'gulp' and 'gulp watch'.

gulp

@@ -0,0 +1,15 @@
+#!/bin/bash -ex
+
+GULP=./node_modules/.bin/gulp
+
+function install() {
+    npm install
+    touch $GULP  # installer doesn't always touch this after a build, so we do.
+}
+
+# Rebuild Gulp if missing or outdated.
+[ -e $GULP ] || install
+[ gulpfile.js -nt $GULP ] && install
+
+$GULP
+exec $GULP watch