add eslint to lint app.js

[cyberhck]

Right now I can see a lot of inconsistencies on code style, it's a minor (but important) thing, we should at least lint app.js for code styles.

Doing that should be fairly simple, just npm init, npm i eslint -D, ./node_modules/.bin/eslint --init (not sure about this one, it's similar), then using a code style (preferably google's) should be good.

We could add this to .travis as well.

thoughts? @Solomonic @lalita13008

[wmathes]

if we do this, we might as well switch to TypeScript. Not urgent for now :)