Add script that bumps example app build number

Examples/xx-messenger/bump-build-number.sh

+#!/bin/sh
+
+script_dir=$(dirname $(realpath $0))
+project_dir="$script_dir/Project"
+repo_dir="$script_dir/../../"
+
+if [ -n "$(git -C $repo_dir status --porcelain)" ]; then 
+  echo "Repository has uncommitted changes!"
+  exit 1
+fi
+
+cd $project_dir
+xcrun agvtool next-version
+build_number=$(xcrun agvtool what-version -terse)
+
+cd $repo_dir
+git commit -a -m "Bump xx-messenger example app build number to $build_number"