[RFC] Split react-tools out into standalone repo

I'm going to close this out regardless, but I wanted to get some feedback on the idea before I went all the way. I've been wanting to split out JSX since … forever. This finally does it but keeps it as the `react-tools` package. (ok, it's not actually moved out here, but it shows what would get moved out).

I think what we could do to make sure the React repo doesn't fall out of sync is to set the dependency to the react-tools repo (presumably reactjs/react-tools). We could also then iterate in react-tools and release changes without necessarily staying strictly in sync like we do now.

I `npm link`ed and jest works, as does (at least) `grunt build:basic`. The JSXTransformer build might be broken and it's also not clear where that would live (maybe it could be rebranded to ReactTools.js?)