Add support for treeshaking (react-toolbox#1423)

According to Rollup guide adding `module` field in package.json helps tree shaking aware tools like webpack 2 or rollup import the ES6 modules directly to use the tree shaking feature and reduce the bundle size.

Author: ankurp

package.json

@@ -4,6 +4,7 @@
   "homepage": "http://www.react-toolbox.com",
   "version": "2.0.0-beta.8",
   "main": "./lib",
+  "module": "./components",
   "author": {
     "email": "[email protected]",
     "name": "Javier Velasco Arjona",