feat(lib): updates code to use es6 and promise based async

[mfdebian]

This PR adds a bunch of changes to the core lib files but basically does 2 things asked for in #203:

• [x] Drops support for Node.js below 6
• [x] Add promise support to async helpers

it also makes the code es6 compliant.

I've intentionally not touched any tests (besides replacing var) and not changed anything in the intended behavior of the library.