Compile JSX to direct createElement() calls

Created by: gaearon

Due to how webpack works today with CommonJS, we pay the cost of three object property accesses (ReactWebpackBinding.default.createElement) for every JSX call. It doesn't minify well and has a minor effect on runtime performance. It's also a bit clowny.

We should fix this to compile JSX to something like

var createElement = require('react').createElement

createElement(...)

Could be a custom Babel transform. Could be a transform that inserts _createReactElement into scope and specifies it as the JSX pragma.