Feature/form submit handlers

[the-overengineer]

Adds support for onSubmitError and onSubmitSuccess handlers to the ResetPasswordForm and the ChangePasswordForm.

Fixes #168

[robertjd]

Thanks @Tweety-FER , I can confirm that I can now set those handlers. I did notice an inconsistency that we should handle. In our docs we say that the handlers can be used to pass a different error back, but that's not true in this PR, it just passes back the original error because it's bound.

I'm referring to this example in docs/api.md:

class LoginPage extends React.Component {
  onFormSubmit(e, next) {
    // e.data will contain the data mapped from your form.
    console.log("Form submitted", e.data);

    // To return an error message, call next() as:
    // next(new Error('Something in the form is wrong.'));

    // Or if you want to change the data being sent, call it as:
    // next(null, { myNewData: '123' });

    // If you call next without any arguments,
    // it will simply proceed processing the form.
    next();
  }

  render() {
    return <LoginForm onSubmit={this.onFormSubmit.bind(this)} />;
  }
}

[the-overengineer]

@robertjd Right you are! Changed the behaviour to use the passed error if it exists, otherwise it uses the old error.