Ship merge, mergeInto, copyProperties to npm

This makes sure `merge`, `mergeInto`, `copyProperties` end up in build/modules which then gets packaged up for npm. Is there anything we're missing?

Ideally we should have deprecation notices. But we've never officially supported these. Also, `merge` now uses `Object.assign` so anybody using these without polyfills will have a Bad Time™.

Reviewers: @spicyj @sebmarkbage @syranide 

Test Plan: `grunt build` and make sure files are in `build/modules/npm-react/lib` (before: they weren't)