Generate bundle and assets when building via Xcode

[frantic]

This integrates React Native into Xcode's build process, which lets us automatically handle bundling and packaging image assets.

Tested the script via https://github.com/frantic/ReactNativeAssetsExample

Loaded from packager:

Loaded from offline bundle:

Android:

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @foghina, @javache and @frantic to be potential reviewers.

[brentvatne]

@frantic - so all we need to do is put images in /assets, no more thinking about Images.xcassets?

[frantic]

@brentvatne almost. Documentation coming soon. That's something we've been working on for a while, it just has pretty big surface area. The idea is yes, we provide a cross-platform alternative for Images.xcassets - you put your images next to your JS files and require them as you'd require JS file (see this). And behind the scenes packager will figure out what files are needed and we'll bundle them using Xcode (this PR) and Gradle (#3462) hooks.

[frantic]

You will still be able to use Images.xcassets or Android drawables if you want to, but this means you'd have to maintain them yourself. require('image!stuff') will be deprecated in favor of good old {uri: 'stuff'} and if you use it – it's going to be your responsibility to make sure those images exist and packaged properly. The only benefit I see for this system is when RN is inside a hybrid app and shares images with it.

We are trying to make the new system ("packager managed assets") available for the next release cut.

[frantic]

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@frantic updated the pull request.