update example.js
cleanup: rename createError to createErr
cleanup: add istanbul ignore
add isOk and isError
updated the README with better example & docs
Merge branch 'improvements' into 0.1.0-improvements

* improvements:
  updated the README with better example & docs
  add isOk and isError
  cleanup: add istanbul ignore
  cleanup: rename createError to createErr
  update example.js
0.1.0-improvement3