Upgrade to ES6 and drop support for Node version < 4

[soda0289]

Currently this parser still supports Node version 0.10 and uses ESLint v2 for linting. We should upgrade the codebase to ES6, using const and let, and stop using the assign library when the native Object.assign() can be used instead.

[JamesHenry]

I'm working on this as part of my work today, because more and more changes are becoming more difficult as a result of our current setup