Add syntax checker for JavaScript using ESLint

For information about ESLint, See https://github.com/nzakas/eslint.
Improve docs of flycheck-eslint-rulesdir
Update changelog
[Fix #291] Merge branch 'yasuyk-eslint'

* yasuyk-eslint:
  Update changelog
  Improve docs of flycheck-eslint-rulesdir
  Add syntax checker for JavaScript using ESLint