Added a new task form component.
Added a new task form component.

Nothing new here. I use two kind of actions to achieve my goal - first
are
TodoListActions (addTask), the second are AddNewTaskFormActions
(changeContent and clearForm).

There's an interesting approach to avoid if's in validationClass method.
It is very similar to 'pattern matching' technique, commonly used in
functional languages.

It's also worth noticing, that if you are keeping the form semantics
(onSubmit vs. onClick), you need to be careful about other forms of
submitting data than clicking the submit button. Notice the check of
this.state.submittable in the submit method. There was a subtle bug that
I could submit an invalid task even if submit button is disabled when I
enter my content and press Enter.

I've used Input and Button from React-Bootstrap package to achieve
great-looking inputs. They also greatly simplifies handling visual side
of validation - I can set a proper class and message when something goes
wrong using properties.

arkency/react_flux_alt_immutable_todolist,arkency/react_flux_alt_immutable_todolist