Upgrade to ES6

[stevenle]

Codebase needs to be modernized to ES6 modules. Modules should export only whatever's necessary for public use to support optimizations like tree shaking.

Clients will need to upgrade their build systems to compile the ES6 syntax into whatever output they desire, e.g. using Babel.

Docs need to provide examples on how to import modules and how to integrate with common build systems.