Make a decision on ES2015

[area]

ethereumjs-vm as of somewhere around version 2 now uses ES2015 language features. As a result, testrpc>=3.0.2 also does. This prevents them from running on node 5.x. It seems shortsighted to pin ourselves to these lower versions as future hardforks will surely e.g. change opcodes or introduce new features that users will want to be able to cover.

I see no reason to not move to ES2015 as a result. Doing so seems like a fine time to introduce an .eslintrc file. I am also tempted to think about introducing async/await, which I've grown very fond of in other codebases.

[area]

This is what I've been using elsewhere, so....

{
  "extends": ["airbnb/base"],
  "plugins": ["mocha"],
  "env":{
    "meteor": true,
    "es6": true,
    "commonjs": true
  },
  "parserOptions": {
    "ecmaVersion": 6,
    "sourceType": "module"
  },
  "rules": {
    /* General  */
    "consistent-return": [0],
    "complexity": ["error", 6],
    "eol-last": [0],
    "eqeqeq": ["error", "smart"],
    "max-len": ["error", 150, 2],
    "no-console": [0],
    "no-mixed-requires": [0],
    "no-native-reassign": [0],
    "no-param-reassign": [0],
    "no-use-before-define" : ["error", { "functions": false }],
    "no-useless-rename": ["error", {
      "ignoreDestructuring": true
    }],
    "no-undefined": ["error"],
    "no-underscore-dangle": ["error", { "allow": ["_id"], "allowAfterThis": true }],
    "object-curly-spacing": ["error", "always", {
      "objectsInObjects": false,
      "arraysInObjects": false
    }],
    "object-curly-newline": ["error", { "minProperties": 1 }],
    "object-property-newline": ["error"],
    "prefer-template": [0],
    "quotes": [2, "single"],
    "radix": ["error", "always"],
    "require-yield": [0],
    "semi": [2, "always"],
    "no-unused-vars": 0,
    "import/extensions": [0],
    "arrow-parens":[2, "as-needed"],
    "no-plusplus":[2, { "allowForLoopAfterthoughts": true }],
    "no-bitwise": [2],
    "import/no-unresolved": [2, { ignore: ['\.sol\.js$'] }],
    "class-methods-use-this": [2],

    /* Plugins */
    "import/no-extraneous-dependencies": [0],
    "mocha/no-exclusive-tests": "error"
  },
  "settings": {
  }
}