Replace jshint and jscs with eslint

[duckontheweb]

Update avn, avn-nvm, and avn-n to use eslint for JavaScript linting.

References comment in #66

[duckontheweb]

@wbyoung How would you feel about using the Airbnb rule set with eslint? This seems to be a pretty commonly used standard and a little more in line with some of the other standards out there than the eslint standard defaults.

[duckontheweb]

Sorry, should have looked through the jscs configs more before asking that question. It looks like you're already using Airbnb as a template. I'll work off of the existing style rules for jcsc and adapt them for eslint.

[wbyoung]

@duckontheweb yup, sound great!