Migrate the build to webpack 2.

[zinigor]

This is a WIP. Needs a companion PR to "work": Automattic/dops-components#89

Changes proposed in this Pull Request:

• Upgrades webpack to 2.2.1.
• Upgrades Babel to 6.13.
• Upgrades node-sass and sass-loader to use the newest libsass version.
• Uses native module loading to allow for "tree shaking" for unused libraries elimination.
• Cleans up webpack configuration.

Still to do:

• Actually make it work because add-module-exports plugin no longer work as it has before.
• Add minification using the new uglifyjs plugin interface which wasn't working before, and remove uglifying from gulp.
• Bring back hot loader configuration that is currently removed due to new Webpack config layout.

[samhotchkiss]

What's the reasoning behind this?

[zinigor]

The main reasoning is to make the client-side code of Jetpack smaller. I don't actually know how smaller we can make it, but if a simple toolchain upgrade can give us that, I'm thinking we should do it. There's a good article about the general benefits here, plus there's a PR we can look at for inspiration: Automattic/wp-calypso#10799

[samhotchkiss]

That's all very cool, and is something we should probably dig into when we do our "shrink jetpack" sprint, but right now we have higher priorities (like getting the settings UI finished) and shouldn't be spending precious dev time on this direction. Closing for now, but I'd love to investigate this more in the future (and let's chat before we do)