Add eslint

[danyshaanan]

WIP

Mainly, we should wait until eslint (and grunt-eslint) will formally support es6.
http://eslint.org/blog/2014/11/es6-jsx-support/

In the meanwhile, here is a test of the current state:
https://github.com/danyshaanan/columnify/tree/eslint

By setting eslint with:
https://github.com/danyshaanan/columnify/blob/873cc81ae66ad4f432511544c0fc95bcb395b8f4/.eslintrc
and running it on width.js, utils.js, and the generated columnify.js file, gives this error count:

$ grunt | grep error | grep -oe ' [a-z-]*$' | sort | uniq -c | sort
   1  consistent-return
   1  eqeqeq
   1  guard-for-in
   1  no-extra-strict
   1  no-return-assign
   1  space-infix-ops
   1  strict
   1  wrap-iife
   2  block-scoped-var
   2  no-loop-func
   3  no-unused-vars
   3  space-after-keywords
   5  no-eq-null
   5  no-shadow
  21  curly
  23  quotes

quotes, which is set to 'single', has a similar number when set to "double".

See meaning of errors here: http://eslint.org/docs/rules/

Adding 'test/*/.js' to eslint results in this error count:

   1  consistent-return
   1  eqeqeq
   1  guard-for-in
   1  no-extra-strict
   1  no-return-assign
   1  space-infix-ops
   1  wrap-iife
   2  block-scoped-var
   2  no-loop-func
   3  space-after-keywords
   5  no-eq-null
   9  no-shadow
   9  no-unused-vars
  22  curly
  33  no-comma-dangle
  64  strict
  79  quotes