Add script to build process during init

[satya164]

Should replace React Native's packager CLI in gradle and xcode scripts with haul during init.

• iOS ~ @grabbou
• Android ~ @satya164

[grabbou]

There's a problem on first Xcode run as haul-integrate.sh will be executed after React own build scripts are executed. That means the very first run will launch packager no matter what we do. Second works properly. Figure out how to fix it.

We might need to run haul-integrate.sh during init command to make the first change. The purpose of it to sit in build script is to avoid updates of React / npm installs.

[grabbou]

Now that we have ENV variables, we will be removing this legacy workaround. See #234 for details.