introduce babel compilation to web-ready es modules, which are made
available in the "dist-esm" directory

summary of changes:

 [.gitignore] modified
  added "/dist*" to ignore the "dist-esm" directory

 [babel.config.js] added
  configures the babel esm compilation

 [index.html] added
  use `npm start` and browse to http://localhost:8080
  to check that the es module is web-ready
  an automated build step would be better

 [package.json] modified
  move scripts block closer to top of package.json
  add field "module", directs browsers to the es module distribution
  add "dist-esm" to the files array
  add script "prepare", runs build and tests on each install/publish
  add script "start", runs local http-server to test index.html
  add script "clean", deletes the "dist-esm" directory
  add script "compile-esm", babel generates es modules in "dist-esm"

- build was failing under node 4
- no reason to support old node for *build* time