Modernize codebase with newer JavaScript syntax, techniques, etc

[KrisSiegel]

msngr.js has been around for quite a long time. When it was initially created it was named intents.js and was used pretty much exclusively in an environment without access to ES6, transpilers or really anything modern. It was almost 2 years before intents.js was renamed and open sourced onto github + npm. This project has carried around that old baggage for quite a while and it's time to modernize a little bit.

This issue is for exploring and implementing multiple modernization efforts. Multiple pull requests will be expected and this will get rolled up into a new, major version release.

• Drop support for node 0.10, 0.11, 0.12, 4.x, and 5.x
• Revise code to drop var essentially everywhere
• Revise code to start using arrow functions where applicable
• Revise code to support ES6 style imports. We must ensure the global availability will continue to work as well as node module inclusion.