Add a prebuild script for Constructicon

Constructicon will run this just before building the project, so this is
our chance to install the node modules we need and create the
.xcodeproj.

script/constructicon/prebuild

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "$0")/../.."
+
+npm install
+rake create-xcode-project