Add React.version

[sophiebits]

(Resubmitting since #200 got closed and reverted.)

getConfig needs to be a function because grunt.config.data.pkg.version isn't available at the time that grunt/config/jsx/jsx.js is required.

Test Plan:
grunt build, grunt lint, grunt test all work. After building, both react.js and react.min.js contain the version number.

[sophiebits]

Just updated based on IRC:

zpao: balpert: i'm going to make an executive decision and do your version diff, but a little differently. i think we should have a separate block which sets React.version = __VERSION__ (outside of the React= { ... })

@zpao I left the jsx config refactoring because in #200 you said "I wanted to get rid of those json files anyway." but I can get rid of it or split it out if you prefer.

[zpao]

👍

@benjamn Are you game to remove those config files?

[zpao]

And that's the sound of the buzzer you hear. Going to do this. We'll just have to keep an eye out for merge conflicts here but I don't think it'll be too much of an issue.

Wishes do come true @tomocchino...