Better build system + browser client-only build

[SEAPUNK]

Self-explanatory.

• [x] reconfigure babel
• [x] webpack
• [x] use es2015 modules
• [x] use es2015 versions of dependencies
• [x] lib/
• [x] package.json has lib/ as main
• [x] browser/
• [x] browser/lib/
• [x] package.json in browser/ has lib as main
• [x] browser/socketeer-browser-commonjs.js
• [x] browser/socketeer-browser-global.js
• [x] browser/socketeer-browser-amd.js

[SEAPUNK]

I can't use import statements for the time being, because nodejs will complain about them.

I'll set up a separate build script for node.js modules.