Add support for multiple input elements in a form-group fixes #22 and #35

[jantimon]

The attached pull request turns the boolean blurred into a map blurred[inputName].

After a change / blur event the isValid and the isBlurred helper functions check if all elements are blurred and sets/removes the error class.

The previous tests weren't modified and are still running.
Added tests for the new feature.

Fixes #22 and #35