Added "module" entry to `package.json` in order to support ES modules with the latest Webpack.
This should enable tree-shaking support ouf of the box for Webpack.
Rollup will continue to use the "jsnext:main" entry to my knowledge.

For more info see https://github.com/dherman/defense-of-dot-js/blob/master/proposal.md
Related issues: webpack/webpack#1979, bvaughn/react-virtualized/issues/427
